Alan Quinn reports on a compression spring gauging system which is now available off the shelf for the first time

Previously only available through Cadar’s bespoke gauging service, the company’s spring compression gauge is now available as a standard product for the first time. Although primarily designed to measure car suspension springs, the gauges can be used to measure a large range of compression springs.

Wherever possible, the approach is to measure components in the same manner as they are used in operation. The gauge has the ability to measure loads in seven axes at any point between free condition and bound state. it can also scragg (pre-stress) springs to loads of up to 25kN.

The spring compression gauge consists of a support frame, a vertically mounted electrically powered ram with an interchangeable spring locator for the major spring bore and a table with an interchangeable locator for the minor spring bore. The movement of the ram is monitored by a glass scale linear encoder and the loads are measured by a strain gauge, upon which the minor bore locator is mounted.

The spring compression gauge has been designed to be very strong and robust, thus allowing it to operate in a shop floor environment. Safety is a prime concern and it is for this reason that the operating area of the gauge is fully enclosed by three clear fixed panels and a sliding door. This door must be closed and for reasons of safety the gauge will not operate with the door open.

The gauge control software is configurable by the user to allow for custom details to be added. Three proprietary software packages are available for use with the gauge: Variables, Checkmate or the new QS-Pro package. Measurement details are displayed on the Cadar Checkstation industrial computer which is supplied as part of the kit.

• Cadar

November 1999

Disc springs made from polymers are an innovative new addition to the Igus product range.

Polysorb disc springs are very space efficient and especially suited for a wide range of manufacturing industries where product designs necessitate a short spring length.

Design benefits and advantages claimed for these non-magnetic springs include improved linear spring characteristics compared with metal alternatives and the elimination of running tolerance and end float. They absorb vibration reduce noise and made from insulating materials provide electrical and thermal isolation.

Polymeric materials are also corrosion resistant and require no lubrication. They are relatively light and reduce space requirements. The Polysorb springs can be used up to 80degC and still have a spring force of 8N. In the event of being used for long periods in submerged conditions they can show a spring force of 10N.

The spring length of a disc spring is relatively small. In practice several disc springs are combined alternately as a stack of springs to increase spring travel which increases in proportion to the number of springs used while the spring force remains the same as that of a single spring. To increase the force the disc springs can be combined in parallel.

Dimensions of the part Spring lengths and Forces

• Igus
• Tel: 01327 31366
• http://www.igus.de

May 1998

The IP ratings for enclosures are well documented. But what is the IK Code for mechanical impact? Alan Quinn went to find out.

The IP Code defined in the standard BS EN 60529:1992 degrees of protection provided by enclosures (IP Code) provides a means of specifying the ability of an enclosure to protect its contents from external objects. The standard applies to enclosures for electrical equipment with a rated voltage not exceeding 72.5kV.

Perhaps less well known is the IK Code, which deals with mechanical impacts. It is defined in the standard BS EN 50102: 1995 Degrees of protection provided by enclosures for electrical equipment against external mechanical impacts (IK Code). It provides a means of specifying the capacity of an enclosure to protect its contents from external impacts.

Before the advent of EN 50102, a third numeral used to be added to the IP Code to indicate the level of impact protection – eg IP66(9). Non-standard use of this system was one of the factors leading to the development of the standard, which uses a separate two numeral code to distinguish it from the old differing systems. The standard came into effect in October 1991 and conflicting standards were supposed to have been withdrawn by April of last year.

EN 50102 specifies the way enclosures should be mounted when tests are carried out, the atmospheric conditions that should prevail, the number of impacts (5) and their (even) distribution, as well as the size, style, material and dimensions of the various types of hammer, designed to produce the energy levels required.

* GAMBICA, the association for the instrumentation, control and automation industry in the UK, has published a new colour handbook as a guide to specifiers and users of enclosures for electrical and electronic systems. The handbook covers every aspect of application, materials, standards, ratings and safety. It also details environmental considerations and hazardous areas. A useful list of enclosure suppliers and their individual services is also included.

Table 1: IK Code and impact energy

< td>0.5

IK Code	IK00	IK01	IK02	IK03	IK04	IK05	IK06	IK07	IK0 8	IK09	IK10
Impact Energy	*	0.15	0.2	0.33	0.7	1	2	5	10< /td>	20

Table 2 Impact test characteristics

IK Code	IK00	IK01 to IK05	IK06	IK07	IK08	IK09	IK10
Impact Energy (J)	*	<1	1	2	5	10	20
R (mm)	*	10	10	25	25	50	50
Material	*	polyamide	polyamide	steel	steel	steel	steel
Mass (kg)	*	0.2	0.5	0.5	1 .7	5	5
Pendulum hammer	*	YES	YES	YES	YES	YES	YES
Spring hammer	*	YES	YES	YES	NO	NO	NO
Free fall hammer	*	NO	NO	YES	Y ES	YES	YES

Note: R is the radius of the striking element

February 1998

The ultimate reliability of bearings depends not only upon the quality of the bearing itself but also the manner in which it is installed.

Ball bearings are the most commonly used of all rolling bearings with the single row radial ball being the most widely used. Self-aligning ball bearings usually containing a double row of balls are also a popular choice in a wide range of industries as they are capable of accommodating misalignment of the shaft of the order of 2-3deg.

In order to obtain the best results from bearings used to support the rotor shaft it is common practice for manufacturers of electric motors to employ a waved washer to pre-load the outer race of the bearing taking into account any tolerance build-up within the bearing assembly and allowing for expansion due to heat generation ensuring that the bearings roll truly. In addition the increased load exerted by the waved washer substantially reduces noise and vibration and extends bearing life.

Another benefit of employing waved washed washers in bearing assemblies is to damp out vibration transmitted from other sources when the motor is stationary as external vibration in a loosely assembled bearing housing can result in damage to the inner and outer races of a ball bearing.

It was in the late 1940s when researchers in the USA were seeking a compact and reliable means of creating a predictable load on the outer ring of the locked bearing in an electric motor assembly that attention became focused on the potentially unique characteristics of the waved washer when made in controlled conditions. By fitting a precision waved washer of known performance in the gap between the spigot of the locked bearing end cap and the outer ring of the bearing the shaft could be located and allowance made for expansion. The desired diagonal pressure thus created across the balls from the outer to the inner ring resulted in reduced ball skidding and retained a true ball rolling effect.

The precision pre-loaded waved washer was quickly adopted in the United States and engineers began to study the characteristics of waved washers finding that performance responded to specific formulae. While other means can be adopted for applying an axial load to the outer ring of the ball bearing none is as compact or predictable as the waved washer which responds to a simple straight line curve.

Whilst three-waved washers are highly suited to many applications there is a limit to the load which could be applied and for larger washers more waves are desirable. Higher pre-loads are satisfied by increasing the number of waves and/or increasing the thickness of the spring steel used to make the washers. Four waves will usually give the increased load required but large washers can be produced with the same accuracy having five six or more waves.

Although bearing sizes have been standardised throughout the world and the outside diameter of waved washers has been standardised in turn there has as yet been no agreed standardisation of the internal diameters. The principal difference between UK sizes and those of waved washers made in mainland Europe and the US is the width of rim which influences the internal diameter of the washer. Emmott adopts a slightly wider rim which is said to increase reliability based on independent tests involving severe cyclic compressions. The washers are also pre-stressed in manufacture to improve reliability.

Design Guides

Four spring design guides are available from George Emmott (Pawsons):

Constant Force Springs

Flat Springs

Waved Washers

Clock Type Springs

They are aimed at assisting product and component designers to achieve optimum results and cover applications design considerations load calculations materials and case studies. These guides can also be requested by the web site www.emmottsprings.co.uk.

• George Emmott (Pawsons)
• Tel: 01535 643733

May 1999

Bryan Harris, Managing Director of Harris Springs, looks at the case for improved spring specifications and design.

Any engineering component can easily become the weak spot in an assembly if it is incorrectly specified. Bearings, for instance, have to be specified against a knowledge of working load, speed range and operating conditions. Get these wrong and problems are sure to follow, so few designers are averse to seeking specialised assistance from bearing manufacturers.

Springs are a similarly specialised area of mechanical design. However, it is less likely that technical assistance from spring makers will be sought. As a result, the field performance of a product may fall short of expectations.

Often the fault will lie, not with the spring but with the design specifications to which it is produced. This may well be traceable to a lack of knowledge of constraints that apply to spring design. Indeed, general awareness of the relevant British Standard (BS1726) is lamentably low.

No-one expects designers to be expert in every field but there is a mistaken inclination to treat springs as commodity items rather than engineering components. When a remedy is sought, it results in higher development costs than would be necessary if the help of specialist spring suppliers had been sought during product development.

The basic point, regularly overlooked, is that standard linear drawing tolerances are irrelevant to spring specification. In practice, no spring tolerances should be applied without first considering the operational effect that they will have in the particular application.

As an example we will take a conventional constant rate compression spring, 50mm long, manufactured to give a rate of 1N per mm compression. If the designer specifies a manufacturing tolerance of +/-1mm on length then an acceptable batch may contain springs between 49mm and 51mm length.

This is fine, so long as the implications have been thought through. If deflection to 20mm is required, then the nominal load will be 30N and variation will be +/-3.33 per cent, which may be acceptable. If, however, the application requires spring deflection to 45mm then the load will vary about the nominal (5N), by up to 1N, or +/-20%, which is unlikely to be acceptable in a load critical application.

When we move to extension type springs, an added complication arises in that each requires a certain load to be applied before it will begin to stretch. This load is approximately proportional to the spring index – the ratio of coil diameter to wire diameter – but is nevertheless an additional variable that requires consideration.

As we can see, spring specification is demonstrably not as straightforward as it might at first appear. Reliable, consistent operation is dependant on having available the maximum amount of application data to help design the spring properly. Simply treating it as a commodity will almost certainly lead to problems.

Fortunately, there is considerable assistance available from British Standards to the form of BS1726. As standards go, it costs little but deals with spring specification clearly and comprehensively, and could save a lot of development time. In addition, specialist spring manufacturers like Harris Springs have computer aided spring design packages available which, given the necessary information, can develop a spring design and specify sensible tolerances based on customers data.

With design complete, it is then possible to control the manufacturing process very tightly. On a compression spring in particular, length is crucial and much work has gone into the development of gauging systems. Harris Springs uses proximity type length probes operating in cycle in conjunction with microprocessor based gauging and statistical process control systems. These link in with the machine control systems for automatic adjustment whilst automatic sorting facilities make it possible to guarantee that every spring in a batch will comply with the designed tolerances.

Thus it is possible to develop a spring that will match the customers requirements exactly and then produce it with consistency.

• Harris Springs
• Tel: 0118 934 0024
• Fax: 0118 934 1365

Automotive engineers concentrate on developing efficient engines. MILES DADSON, HBM UK’s automotive engineering specialist, discloses some unexpected areas of development

In order to reduce the fuel consumption of car engines, engineers within the automotive industry are trying reduce friction in the cylinder head. Valve gear is of particular interest to the energy efficiency lobby because this part of the internal combustion engine is a major contributor to engine friction at low speeds. In order to compare different designs and various combinations of materials, the cylinder head is separated from the engine and driven by an electric motor. Friction is then calculated by measuring the torque required from the electric motor to operate the valve gear.

In a series of experiments carried out in Germany, at the University of Heilbronn in conjunction with the Steinbeis Transfer Centre, a number of issues have been raised about conventional cylinder head testing configurations. Test rigs that use torsion shafts and supported couplings have shown their limitations over new flange-based transducers.

The first concern is a conventional test rig’s susceptibility to damage by unforeseen loads. The forces exerted by the valve springs give rise to very high alternating torques in the camshaft. In a typical set-up, when testing over the whole cylinder head, the average operating torque will range between 1 and 25Nm, but peak torques of over +/-150Nm can occur due to the valve springs. When the University technicians took measurements with conventional transducers, frequent damage occurred to both the shaft and the curved tooth couplings because the peak torque loading was well outside the shaft’s specification.

To overcome the break-downs, the technicians tried fitting a centrifugal mass on the cylinder head to even out the dynamic loading on the valve gear. This addition smoothed the torque peaks but created more problems. The large rotating weight posed an additional risk to those in the test environment and changed the quadratic response of the measuring equipment.

Even with the centrifugal mass, damage repeatedly occurred to both the curved tooth coupling and transducer shaft. To eliminate this maintenance burden, the technicians needed to develop a tougher set up that could handle the torque peaks.

Simply using larger specification shafts and couplings is not sufficient. While larger units would be able to handle the transient torques the device will be less accurate at measuring smaller changes in torque that are required in different tests. The unit would also be physically larger and may require test bed modification.

As well as monitoring the performance of the complete cylinder head, testing technicians can employ strip measurement, where certain components are removed from the cylinder head. The friction contribution of the removed components can then be inferred by comparing the stripped data to the data collected from the complete test rig. For example, if the camshaft only is measured, without actuation of the valves, then torque values occur only in the range 0.5 to 1Nm, and there are no torque peaks. In these tests the equipment needs to be sensitive to smaller changes in torque to obtain meaningful test data.

The University’s technicians were presented with a dilemma. How to provide a torque measuring device that could withstand the rigours of testing the complete cylinder head at the same time as having something sensitive enough to carry out strip measurements, without changing the transducer between tests? They turned to a new type of flange-based transducer from HBM. The device, a T10F, has a very short overall length, allowing the flange to be mounted conveniently without making lengthy and expensive modifications to the test bed during a test programme.

The torque, speed and position-measuring flange consists of an inner and outer ring connected by four ‘I’-section spokes and housed inside a free standing split-ring stator. The spokes make the device up to 30 times stiffer than the conventional transducers previously used by the University. The T10F can withstand 160% of the test rig’s nominal torque, compared to the 70% offered by conventional transducers.

• HBM

July 2000

This guidance from Sarel is aimed at users of electrical switchboards, but it is equally applicable to the housing for any major item of electronic equipment.

Miniaturisation of components, the widespread use of electronics and the appearance of new power electronic base products make temperature management a constraint similar to IP, which must more and more frequently be taken into account in the design of electrical and electronic housings.The life of a component such as a switchboard inside a large enclosure depends upon the temperature and humidity conditions. The ideal temperature is between 10 and 45degC, with a relative humidity of 30-90%. There are various solutions. In some cases, it is sufficient to oversize the enclosure and use fans. In more extreme cases where the ambient temperature is very high, the refrigerating principle (cooling units) must be used by installing air/water exchangers or air coolers.

Natural ventilation

A supply of cool outside air through the ventilation louvers can improve heat dissipation by natural convection. However, this solution can only be used when the dissipated power is low and when the environment is not polluted by dust.

Circulating

Using a fan to circulate air within the enclosure can even out the temperature and prevent hot spots which could be harmful to some components.

Forced ventilation

Many fans are designed to remove large quantities of heat generated by components in electrical switchboards. The increases component life span significantly. They provide an efficient solution and are easy to implement and maintain. They can be used in industrial settings as well as in services

Cooling by air/air exchangers

Warm air within the enclosure and cool air from the surroundings are made to flow by two fans. They circulate on either side of sealed separation walls which stop dust and humidity from getting into the enclosure. Warm air from the switchboard heats these walls, which are cooled by the colder outside air. The transfer always takes place, of course(!), from the hotter side to the colder one, which is why these devices can only be used if the ambient temperature is lower by at least 5Cdeg than the required inside temperature.

The aluminium exchange cassette is the heart of the system. This element is easily removed and can be cleaned very easily. The continuous running of an internal fan prevents hot points in the switchboard. The temperature control system of the device turns the external circuit fan on or off.

Air/water exchangers

The working principle here is the same as in air/air exchangers: the cool air is replaced by cold water supplied by a network installed by the industrial facility. Due to this change of fluid, much larger quantities of heat can be removed and temperature within the enclosure can also be decreased below the level of the ambient air. The temperature within the enclosure is controlled by modulating the air flow. The water circuit is protected by a supply shut-off device.

Cooling units

Cooling units can be used in the most severe environments, where temperatures may be as high as 55degC. These units are particularly suitable when the required ambient temperature in the enclosure is below the ambient temperature or when large quantities of heat are to be removed. As in the case of exchangers, they do not affect the switchboard IP.

Thanks to the filter located at the inlet of the outside air circuit, they can work even when the ambient air is loaded with dust or oil particles. The filter is easy to replace and inexpensive. Units have an enclosure temperature control system and an alarm function which is set off in the case of improper working.

The thermal balance, whereby the power dissipated by the equipment is compared to the power which is spontaneously exchanged by the walls of the enclosure, is used to calculate the temperature reached within the enclosure when there is no temperature control device. With a knowledge of the dimensions and thermal characteristics of the enclosure, the power dissipation by the operating components and the likely characteristics of the ambient air, it is possible to determine the maximum and minimum internal temperatures likely to be reached without a temperature control system. This is fully explained in the Sarel brochure Thermal Control for Electrical Switchboards.

• Sarel
• Tel: 01793 514774
• Fax: 01793 533046

Many difficult measuring problems can be solved by using balls of known size and accuracy. Alan Quinn looks at the latest from Spheric Engineering.

Think of where precision balls may be used and you generally come up with ball bearings. Which is quite reasonable, even though the price which manufacturers have been able to command for this application has fallen steadily over the last generation. Equally, a mind-boggling five billion balls for ball-point pens is manufactured every year!

But the latest trend for specialist ball manufacturer Spheric Engineering is to move more into added value systems and assemblies involving its high accuracy metal and ceramic products. Many difficult measuring problems can be solved by assemblies using balls of known size and accuracy.

Master balls are used for measuring the location of internal grooves, the depth of holes, internal tapers, flatness and parallelism of micrometer anvils and setting external gauges and comparators. Calibrated master balls are manufactured primarily in tungsten carbide. Calibrated balls have their uses mainly in metrology and standards laboratories, inspection departments and tool and gauge rooms. Because they are manufactured for use as reference standards, Spheric’s high precision ball sets are designated master balls.

A certificate of calibration is supplied with each master ball set, which provides the actual diameter and sphericity read in one plane. Also, a premium certificate of calibration is available, which provides the actual diameter and sphericity readings in three planes on each ball. The certificates are traceable to NAMAS (ISO 9000) and NIST.

A computer-controlled measuring machine (CMM) is a computer-controlled, three-axis system capable of repeating accurate inspection of high volume and complicated components and storing the data for quality control purposes.

The qualification sphere for a CMM is used to calibrate and set up the measuring stylus or probe and set the home/zero positions of the machine’s three measuring scales ready for use. The data from this initial set-up is stored in the computer to enable the program to control the table movements accurately and take measurements from the machine scales, the positions of which are determined when the measuring stylus or probe contacts the surface of the feature being measured.

Generally, the spheres are made from tungsten carbide, steel or zirconia, in a diameter range from 0.375in to 2in. This is mounted on a stainless steel stem of length between 50-100mm and having a diameter relevant to the size of the ball. The stem has a metric thread at the opposite end to where the ball is mounted, which is used to locate and secure the qualification sphere to a machine table of the CMM machine. The ball is mounted using a high strength adhesive and the joint is tested to a load of 20kgf. Variations include imperial and metric versions, cranked stems, and a reference ball with wire and id tag

Unusually, the joint strength is kept intentionally low to ensure that failure WILL occur at moderate loads. This serves as a means of limiting or preventing damage to expensive CMM equipment. Equally, in the event of a severe impact loading which may distort or damage the ball, failure of the joint acts as a positive indicator.

With a hardness of approximately 1500HV, high stiffness and very good dimensional stability, tungsten carbide is an ideal material for reference standards. Its high wear resistance ensures minimum deterioration in use and allows the time between recalibrations to be maximised. Its coefficient of expansion of 6 x 10^-6and low rate of thermal conductivity ensure a low temperature sensitivity and allow use at normal working temperatures with minimal error.

Spheric’s tungsten carbide reference balls are supplied with a NAMAS Certificate of Calibration, which lists the absolute diameter and sphericity of the ball as measured in three planes. Roundness traces are provided for each plane and all measurements are traceable back to National Standards.

There are two tolerance levels:

Grade 10: measured in three planes, the ball may exhibit a maximum deviation from a true sphere of 10µin.

Grade 5: measured in three planes, the ball may exhibit a maximum deviation from a true sphere of 5µin.

For reference tooling balls, AISI 440C stainless steel is sometimes specified for less arduous applications where the properties and cost of tungsten carbide are not warranted or justified, As with all steel gauges, they must be handled and used with care and replaced or recalibrated more frequently than their tungsten carbide counterparts.

• Spheric Engineering
• Tel: 01293 541377
• Fax: 01293 511921
• Contact: Julian Pratt

Early composite springs foundered on manufacturing costs. But can they make a comeback. Dr RAYNER MAYER of Sciotech thinks they can.

The sprung suspension is vital to the ease of ride and safety of road vehicles yet the design of the spring has changed remarkably little over the years. Traditionally suspension springs have been made of steel a strong inexpensive material the properties of which are well understood. Its disadvantages are its weight and its inflexibility unable to bend very far for a given amount of energy absorbed.

Given that transport usage is increasing throughout Europe resulting in higher levels of noxious emissions vehicle weight is becoming even more of a major issue. The Eurosprings solution to the problem is to make the springs from glass reinforced polymer (GRP) a lightweight versatile material that possesses the strength of steel but is one-fifth the density.

A spring made from GRP can be moulded to its final shape in a single operation unlike one made from steel which requires multiple forming and heat treating operations. The lighter weight composite spring therefore offers the prospect of significantly reduced fuel consumption. Also because it is more flexible and resilient the spring absorbs more energy resulting in a more comfortable ride.

The original concept of GRP springs was first developed by GKN but the design and manufacturing process proved too expensive and the company went out of business in 1991. It was at this point that the Eurosprings project’s lead partner Sciotech entered the scene. The company concentrated on developing a flexible manufacturing method which would lower the production costs.

Within a EUREKA project the company gathered together a team of materials suppliers GRP moulding companies research specialists and end users who between them possessed the expertise and resources to design and test the prototype springs devise a cost-effective manufacturing method and develop a product which would be received enthusiastically.

According to Dr Rayner Mayer Sciotech’s chief consultant: “The major breakthrough came in developing a process which allowed one of the partners Fleming Textiles to pre-form the spring reinforcements by producing a glass pre-form package of identical shape to that of the spring. All the quality control can be carried out on the pre-form and when you mould it you simply cast the resin around the reinforcement rather like reinforced concrete.”

Eurosprings is a complex project to run. Mayer has the challenging role to keep all 16 partners on track with an ambitious production and testing schedule. The project has only one academic research member Reading University the others being involved in industrial development and applications.

The springs are currently being developed for London taxis and road haulage trailers prior to commercial production. Then logical next phase is to develop a composite GRP spring for passenger and freight rail vehicles. The DTI has contributed 50% of the costs towards a feasibility study for the new project designated Eurobogie enabling Mayer and his team to travel throughout Europe to evaluate existing technology mark out the potential market and start shaping the project itself.

The team have already lined up the launch customer EWS which is a subsidiary of the American-owned Erie and Wisconin. This provides a valuable entree into the North American market.

* Tractor suspensions from Reyco Industries can now be spec’d with lightweight composite springs. The Model 102 suspension can be ordered with Liteflex composite springs from Delco Chassis Systems. The suspension is said to be 350 pounds lighter than comparable multi-leaf steel spring suspensions. The composite springs are said to last three times longer than conventional springs and provide better ride quality.

* Readers wishing to study the subject further are encouraged to reviewAutomobile Ride Handling and Suspension Design With Implications for Low-Mass Vehicles by Robert G Riley

Rayner Mayer
Sciotech

01252 873564

Eurosprings:

Project Duration 4.5 years
Total Cost MECU 4.10
Participating members: UK, Czech Republic, Denmark, Hungary, Latvia, Netherlands

Main partner/project leader:  Sciotech Consultancy

Partners include:

Reading University
London Taxis International
Plastech Thermoset Tectonics
Polymarin BV
Polymath Engineering
Institute of Polymer Mechanics
Meritor HVS
Brecknell Willis Composites
SVUM
Vetrotex
Autokut
Culzean Fabrics
Don-Bur (Bodies and Trailers)
EM-Fibreglass A/S
Flemings Textiles
Scott Bader

Eurobogie:the next phase of the project is currently seeking new partners

May 1998

With their dated technology, even the best speakers are the weak link in any Hi-Fi system. But here’s a digital alternative!

If the company 1…Limited has its way, there is going to be a revolution in loudspeakers in the next few years. Conventional loudspeakers are the weakest link between recording sound and reproducing it – they are still based on magnets and moving coil technology invented in the 1920s. 1…Limited is developing a revolutionary digital loudspeaker which will incorporate new transducer technology using helical springs made of piezoelectric ceramics.

The company defines a Digital Loudspeaker (DLS) as a speaker which takes digital signals, for example from a CD player, as its input and emits digital pulses of sound. It contains no electrical digital to analogue converter: that task is left to the human ear. Speakers using piezoelectric ceramic springs will be able to project three dimensional sound across a space and do it in a way that can ‘steer’ a sound in a particular direction.

Tony Hooley at 1…Limited came up with the idea of using helical piezoelectric ceramic springs in digital speakers. Quartz is a natural piezoelectric material used in digital watches and computers to produce stable vibrations at high frequencies. Polycrystalline ceramics, such as PZT (lead zirconate titanate) can be made to mimic the behaviour of natural monocrystalline materials by polarising the crystals within the ceramic.

An electric field is applied at high temperature to align the microscopically small piezoelectric domains within the material. This produces a net polarisation in the direction of the electric field which, when a smaller electric field is later re-applied to the material, causes a strain in the direction of the polarisation. In traditional speakers, these materials are used in transducers to convert the digital pulse into a vibration that expands and compresses the air to form an audible sound wave. The problem lies in the manufacture of helical springs from this material. Such a complex shape couldn’t be machined from a solid block, for example. Hooley found a partner in Dr David Pearce from the University of Birmingham, whose experience in the field of piezoelectric materials gave him the confidence to build the helices.

The helical springs start in powder form mixed with polymer and solvent. High temperature metal electrodes are applied, simply using screen printing techniques. The plastic material can then be formed into virtually any shape. As the solvent dries, the material solidifies and a certain amount of predictable shrinkage occurs. The polymer is then removed through slow heating to 600degC, followed by sintering of the remaining ceramic at between 1100 and 1200degC.

Good vibrations

The spring coils around an inner section which contains a novel low-friction contact bearing and a moveable core. The coil tries to coil or uncoil when an electric current is applied, affecting the pressure inside the linear bearing. If the coiling is greater at one end, the core will be forced to move along the axis of coil in line with the applied voltage. This is rather like a hand squeezing a wet bar of soap. The movement generates a vibration of the air next to it and therefore produces noises.

When an array of these devices are placed together they can produce a unique way to shape sound. Not only can they produce a coherent sound image away from the speaker, but the transducers can be driven by clever digital processing, so sounds can be aimed in a particular direction. This will have applications in PA systems, as well as audiovisual areas like cinemas. The prototype has already been built to prove that the ceramic helices work in the way the researchers expected them to, but there is still some way to go. The small transducers will have to be mass-produced in a cost-effective way. Besides the springs – and there are companies working on other aspects of the design – the low-friction contact bearing which transmits force from the helix to the core must be able to move very quickly. The current design being developed by Ranier in Cambridge uses an inflated tolerative polymer, rather “like a balloon rolled in on itself,” as Pearce describes it. But the material needs to be developed further.

1..Limited and the University of Birmingham have teamed up with a number of other companies and academic institutions in a Framework 5 European Programme, PADS, to develop this exciting technology to a fully operational prototype in a couple of years time. Marketable speakers should follow a couple of years after that, offering pure digital sound.

For further information, the University of Birmingham IRC in Materials can be contacted on: 0121 414 7836, and 1…Limited is on: 01223 422290.

• University of Birmingham IRC in Materials
  Tel: 0121 414 7836
• 1…Limited
  Tel: 01223 422290

May 2000