Built to a modular concept Mayr Transmissions’ Smartflex coupling is optimised for the requirements of servo drive lines and offers the machine designer the optimum flexibility with minimum stockholding.

Servo applications for direct drive lines are very common today. The vast increase in quantity has made the drive lines more and more economic. This has called out for a more economic but still high quality servo coupling.

The Smartflex coupling consists of a steel bellows clamped on shaft bushings by clamp collars. For installation the parts are put together and frictionally locked by clamping just one bolt each side. Hence blind assembly is possible. The clamping ring hubs are locked to the shaft frictionally by tightening the clamping bolt. This assures a backlash-free torque transmission between input and output shaft.

The modular concept allows a number of different components to be stocked without needing to stock a variety of complete couplings with different hub bores. Different shaft diameters are accommodated by a range of hub bushings of different bore.

The multiple layers of the steel bellows of the Smartflex provide extremely high torsional stiffness yet still compensate for larger shaft displacements with low restoring forces. This ensures true positioning of the drive and protects the shaft bearings from premature failure due to excessive radial loads.

Made from aluminium the hubs together with the light metal bellows provide an extremely low moment of inertia making the couplings suitable for highly dynamic fast reversing drives.

• Mayr Transmissions
• 01535 663900

Need a specialist bearing, but don’t know where to turn? Tommy Miller looks at a new range that seems to fulfil most unusual needs

Specialist bearings have traditionally come from a variety of sources, depending on whether the need is for resistance to high temperatures, corrosive fluids, vacuum, super-clean conditions, nuclear, non-magnetic components or high speeds. But there is now a range, known as Spacea, that fulfils all of these needs. Indeed, this range covers not only ball bearings, but also linear guides and ballscrews.

Developed by NSK-RHP, the Spacea range includes products that utilise high performance materials and coatings, and specialist lubricants have also been introduced. For example, as well as hybrid bearings with steel races and ceramic balls, there are also all-ceramic bearings that can be used in corrosive environments, air or vacuum, at temperatures up to 200degC. For other applications under vacuum, it may be appropriate to use the products that have gold, silver, lead or molybdenum disulphide solid lubricants. If the need is for a bearing to operate under vacuum and at high temperatures (up to 500degC), silver is the preferred solid lubricant.

One of the new lubricants that has been developed by NSK is the LG2 grease that is suitable for clean-room use. Compared with fluorine greases, LG2 is claimed to provide 10 times the life as well as superior corrosion protection. Depending on the other conditions, it may be better to use fluororesin-coated bearings and ballscrews, perhaps if there is also a vacuum and high temperature – as might be found in production lines for liquid crystal displays.

Other coating and plating options available for the steel Spacea bearings include cold fluoride chrome plating for improved corrosion protection, hard chrome plating or nickel alloy coating. This latter coating is said to provide the highest level of resistance to corrosion and attack from most acids, but at a very reasonable cost.

Applications for the Spacea bearings, linear guides and ballscrews will, inevitably, be extremely wide-ranging due to the vast array of materials, finishes and lubricants that are included. Nevertheless, it is fair to say that they are likely to be used in aerospace, semiconductor processing, the nuclear industry, medical equipment and many more.

• NSK-RHP

June 2000

Mayr Transmissions argues that electronic overload protection still can’t react quickly enough.

Electronic devices for overload protection used to be extremely slow, compared to mechanical torque limiting systems, such as the Mayr EAS-NC. With substantial improvements in servomotors and intelligent controls, this gap has been reduced and servomotors today can react much faster than ever before. Lighter and more powerful motors with responsive electronic controls make motors extremely dynamic.

Some manufacturers argue therefore that, with lower inertia of the servomotor, it is no longer necessary to use mechanical torque limiters to protect a machine against the costly damage and downtime caused by jams or collisions. In order to evaluate this situation, Mayr Transmissions has undertaken research, including all the ranges of low inertia motors from the leading manufacturers. Technical data, including inertia and acceleration, has been considered.

The minimum stopping times of motors have been calculated at different inertias on the driven side of the motor. In order to avoid any controversy about the electronic reaction times in the system, zero delay was assumed, which in practice never happens Ð a more realistic value would be 5ms.

The stopping times of the fastest motors on the market have been applied to an actual collision test of a machine.. The maximum torque would occur in this machine after 15-20ms, whereas the fastest motor would take approximately 30ms to come to a complete stop. A mechanical torque limiter reacts in this realistic example in 3ms. This means that when using electronic protection, the maximum collision force that occurs in 10-20ms still applies before the feed drive can be stopped. The damage still occurs.

This of course is only example using a specific machine, but, considering the fact that the more commonly used motors would take 50-60ms to stop the drive, it is not possible for an electronic system to beat an instantaneous mechanical system. Only if there is very slowly increasing torque will an overcurrent relay be adequate.

Any future limitation to reduce motor inertia to a value that would allow faster stopping times relies on the ratio of motor inertia to ballscrew and turret inertia not exceeding 1:2.5.

• Mayr Transmissions
• Tel: 01535 663900
• Fax: 01535 663261
• Contact: G A Harrison (Technical Director)

Alan Quinn reviews Flender Power Transmission’s new technical handbook. Will it become an industry benchmark?

Covering a wide variety of calculations and useful data through 12 sections running over 150 detailed pages, the Flender Power Transmission technical handbook has a place on any design engineer’s bookshelf.

The pages cover almost any topic which a designer of power transmission is likely to need. There are sections on SI units, maths, physics and geometry and the mechanical properties of materials – surprisingly extensive in scope. These are in addition to the more predictable sections on the geometry and load-carrying capacities of involute gears and the properties of cylindrical gear units, shaft couplings and vibration – a highly mathematical treatment listing formulae for the calculation of stiffness and thereby of vibrations.

Flender’s extensive product range includes geared motors and gear units, as well as couplings and clutches, electronics and special design geared motors.

Indeed, Flender might have done itself a slight disservice in that because the handbook attempts to cover such a wide range of topics, the coverage of the company’s own products is actually briefer than you might have expected!

Technical merits

Another minor criticism is that there is also a commercial price to pay in the introductory 16-page colour section – in fact, the only colour section in the publication – which provides all the corporate information, which seems rather out of place in a publication which is otherwise so comprehensive in its technical content. Maybe more space devoted instead to worked examples might have earned the company more credit.

Those criticisms aside, there is no doubt that the handbook will find a place on many shelves – including my own, as a likely source of those elusive formulae and engineering facts which should be on the tip of one’s tongue but never seem to be! An example of this is the set of tables on explosion protection of electrical switchgear.

Highlight of the handbook for this non-practising engineer is from the heart of the company’s core material and covers the geometry of involute gears. The section contains two tables of the most important formulae used for the determination of sizes of a cylindrical gear and a cylindrical gear pair, and this for both internal and external gear pairs. A further table lists the derived quantities which are used in the calculation of load carrying capacity.

• Flender Power Transmission

January 2000

Most brakes simply operate as ‘on/off’ devices, but this often proves to be unsatisfactory. Tommy Miller describes a new intelligent brake from Lenze.

Although conventional spring-operated brakes perform adequately when they are simply required to prevent motion, there is often a need to slow a machine progressively or to use the brake to help control the motion. To solve this problem, traditionally users have been required to use complex multi-stage brakes and associated switching logic.

A new development, however, allows standard spring-operated brakes to be controlled in such a way that the braking torque applied is within the range 20 to 100% of the brake’s rated torque. By taking advantage of the system’s Canbus communications link, the torque applied by the brake can be controlled concurrently with the torque applied by a drive, allowing enhanced motion control to be achieved.

Automatic monitoring

A further advantage of the new system is that brake wear can be monitored automatically, even to the extent that alarms can be raised if the wear reaches a predefined level, or if the number of applications of the brake exceeds a certain amount or the operating time becomes excessive. This all contributes to improved reliability and reduced maintenance – and hence lower operational costs.

Introduced to the UK by Lenze, the system is being described as a mechatronic braking system with Moditorque control. The company’s standard BFK-series spring-operated brakes are suitable for use with the control system, with the brakes’ rated torques ranging from 8 to 400Nm. The Moditorque control system is available in three operating voltages: 24V, 48V and mains voltages. Programming is achieved by means of a Windows-based software package.

Almost any type of automation or special-purpose machinery could benefit from the additional control, depending on the process and whether the equipment or material being processed might suffer from being over-braked. However, Lenze also highlights that fork-lift trucks are an area where the signal from a foot pedal could easily be translated into a progressive braking action on the wheels – with synchronisation between multiple brakes being easily achieved via the Canbus communications.

Escalator drives and cranes both have to carry variable loads, so these two types of equipment could also be improved by the addition of more braking control and the facility for remotely monitoring the level of wear. In addition, cranes could make good use of the synchronisation between the torque supplied by the drive and the torque applied by the brake.

Whatever the application, the Moditorque system seems to bring spring-operated braking a long way forward from the traditional all-or-nothing approach.

• Lenze

May 2000

Some of the most interesting news in power transmission this month has been software related. STUART NIVEN reports

Two gear suppliers are busy using computer systems to help with gear design this month. Holroyd is playing a leading role in developing a radical new method for predicting contact patterns and other details vital to the successful design and manufacture of worm gears.

It means that highly accurate data can now be provided by computer, putting important aspects of production on a much more scientific basis and giving a number of advantages over existing procedures, which are heavily reliant on costly empirical trials.

The development is the result of a study into work gear transmission carried out by Michael Fish (no, not that one), a research assistant at Huddersfield University’s mechanical engineering department. With the backing of five British gear manufacturers, the consortium had the support of the DTI and the British Gear Association (BGA).

Holroyd has now appointed Fish as a full-time research engineer, where he will continue the work and where the system is now being used as a practical design and development tool.

The software quickly generates accurate representations of the final off-load contact conditions that will be achieved using the given parameters. An exact contact-marking pattern illustrating this information can be generated up-front.

The close correlation of the synthesised and recorded marking pattern confirm the accuracy of the new method. The software can calculate – to an accuracy as good as 2-3Em – the transmission error for any gear design, including simulated error courses anticipated by the operator in contact conditions.

So the new system enables the required contact conditions to be achieved more quickly that with existing iterative processes which involve cutting then marking the worm and wheel set, followed by inspection and assessment.

Meanwhile, at a less fundamental level, ZF is releasing a user-friendly CD-Rom to assist machine manufacturers who are considering using the company’s Servoplan range gearboxes, which are of a compact planetary type and feature very high torsional rigidity and low torsional backlash.

Although the range has been designed to be user-friendly, with whole number integer ratios and coaxial input and output, ZF still believes there is merit in going one stage further with helping the designer in the selection process. The program automatically selects the ideal gearbox from input data including output torque. The size of the search range is then selected, to offer as few or as many possibilities as the user requires. Alternatively, a more direct method of selection can be used, by simply choosing the relevant size category and ratio.

Other functions include a menu which lists general information about the range, including efficiency, installation details, mounting positions, degree of protection and so on. An innovative feature is that any information the user finds relevant can be placed on a special clipboard.

• ZF
• Holroyd

October 1999

What exactly is meant by the terms ‘maintenance-free’ and ‘sealed-for-life’ when when they are applied to bearings? Derek Hoult of Ina Bearings explains

Maintenance-free bearings are very good for use in areas that are difficult or hazardous to get at for re-greasing bearings, and they also have the added advantage that the equipment user does not have to be concerned about maintenance programs for the equipment.

When bearings are selected for a maintenance-free application it is important that all data regarding the duty cycle of the equipment is considered. In addition to the loads and speeds, the environment – such as temperature and cleanliness of the surroundings – is also important.

‘Maintenance-free’ bearings are only maintenance-free in as much as they do not have facilities for them to be re-greased. Sometimes they are called sealed-for-life bearings, but this term is slightly misleading and can lead to confusion.

Bearings that are sealed need the correct lubricant to meet the conditions in which the bearings are expected to perform. Because it is the life of the grease that generally determines the life of the bearing, once the grease has lost its lubrication properties the bearing will become damaged and will fail.

The sealed-for-life bearing is only as good as the lubricant that is used. It is also important to understand that grease does not last forever, so neither will the bearing – as is sometimes assumed when specifying sealed-for-life bearings. Users must realise this and specify what ‘life’ they expect from the equipment so that the most appropriate bearing can be selected.

Different bearing types

The most common sealed-for-life bearings are deep groove ball bearings and linear guidance systems but these are not the only ones that are classed as maintenance-free. Rod-end bearings used on hydraulic rams, for example, can also be produced with a low-friction PTFE-type lining material that needs no lubrication. Indeed, it is very important with these that they are not lubricated as this severely reduces the life of the bearing. This is because, to operate effectively as a low-friction maintenance-free bearing, some of the PTFE is transferred from the coating to the inner spherical, providing a PTFE-on-PTFE sliding surface. If grease is applied, the material transfer does not take place and the bearing will actually wear much more rapidly.

As was stated earlier, sealed-for-life bearings are only as good as the lubricant used and this is equally true for bearings that can be re-lubricated. Therefore the benefit of being able to re-lubricate is that the grease quality can be maintained over a longer period and the bearing life will, therefore, be extended. However, this pre-supposes that the required re-lubrication is carried out at the correct intervals, using the correct grease, and that the quantity injected is also correct – which of course is a maintenance management issue.

Sealed-for-life bearings, then, if they are correctly specified, eliminate the possibility of poor maintenance affecting the bearing life and subsequently affecting the product’s expected performance.

• Ina Bearings

March 2000

Bonding PTFE to traditional metal backing plates is the key to a direct replacement for many tilting pad thrust bearing applications which currently use whitemetal (Babbit metal).

Based on a PTFE sliding surface Michell Bearings’ Tetraglide allows much larger specific loads than whitemetal – reportedly up to three times as much. Its potential is being explored initially in large power generation applications. Other benefits include the superior frictional properties of PTFE greater longevity and a bearing surface which is more tolerant of abuse. The smaller working area reduces losses by up to 30%.

The system is highly tolerant to overload and thermal transients. Significant cost savings in new installations can result from the elimination of the high pressure oil injection system.

Michell has recently been working with the First Hydro Company a leading hydroelectric power supplier. Traditional whitemetal bearings could have difficulty coping with extremely high temperatures speeds of 500rpm the strains and the need for changes in direction found in First Hydro’s hydroelectric stations.

Apparently the concept of using PTFE instead of whitemetal for coating bearing surfaces has existed for many years. Pioneered in the former Soviet Union and the People’s Republic of China PTFE has never been used in the West for a number of reasons including rudimentary design and perceived inconsistent performance.

Both Michell and First Hydro knew about PTFE and the advantages it could bring to bearing design such as the ability to operate at temperatures far in excess of those normally requested of whitemetal – whitemetal flows at 150degC but PTFE is thermally stable until about 250degC.

Michell had investigated the use of PTFE for bearings and the company’s designers and metallurgists had developed a way of overcoming the main problem – fixing the material to the bearing.

In the new design a 3mm layer of PTFE is bonded to the steel backing by using a copper wire matrix intermediary which is soldered to the steel substrate. The PTFE forms a mechanical bond with the copper wire matrix.

The reduction in friction is particularly beneficial at start-up as the need for a high pressure oil injection system is no longer a requirement.

Trials conducted in-house on full-size test rigs and in a two-year trial at First Hydro’s Ffestiniog pumped storage plant are said to confirm these advantages. Long-running field trials have been on applications involving thrust collar diameters of around 1500mm.

Michell has also worked with Fuji Electric Japan and has supplied a set to Hikada Station in Hokaido which were commissioned in March this year. Part of the deal with Fuji is to look at the application potential in journal bearings – when combined with PTFE journal pads these bearings can provide electrical insulation against circulating currents.

Smaller applications would be just as successful but do not necessarily offer the same degree of cost saving or energy efficiency and therefore there is not the same incentive to replace whitemetal. However Michell Bearings is known to be investigating filled grades of PTFE and the effects of the fillers on the copper wire matrix.

• Michell Bearings
• Peter Lummis
• 0191 273 0291

November 1998

Flexible membrane couplers exploit the tensile and flexural properties of membranes (discs) pressed from cold-rolled stainless spring steel. Attached alternately to the driver and driven members, the membranes transmit torque in tension, while flexing readily in bending mode to comply with alignment errors.

These heat-treated membranes can sustain an almost infinite number of flexural (misalignment) cycles. Huco-Flex M couplers are rates for continuous unidirectional and bidirectional rotation up to 15,000 rpm and are typically specified for high resolution measurement devices, high gain velocity or motion control systems, position-critical frictional loads and dynamometers.

Typically, Flex M couplers conduct the transmission through two complementary angles when compensating for radial shaft errors. The greater the distance between the membranes, the larger the radial error than can be accommodated. To cater for the differing levels of radial error, Flex M are made in two standard lengths.

Being pivotal devices, membrane couplers are also available in single-stage form. When linked by an intermediate shaft, these make up into “whirl-free” cardans (floating shafts) that extend the coupler’s reach and accommodate larger offsets. Cardans de-mount into three parts and, in this form, Flex M can be fitted or removed without disturbing pre-aligned shafts.

Misalignment

The maximum radial error should be determined under the worst case tolerance accumulations before selecting a short or long coupling. If correct shaft alignment relies on setting-up by skilled factory personnel, it must be able to be replicated by service engineers. Should the predicted radial error exceed the capacity of standard two-stage couplers, single-stage versions should be used linked by an appropriate intermediate shaft.

Resonance

Excitation frequencies of closed-loop position or velocity control systems can give rise to resonance when the loads are predominantly inertial and the load inertia exceeds that of the motor. In these conditions, the coupler’s torsional stiffness should be such that the natural resonance frequency exceeds 300-600Hz, depending on the dynamics.

Torque

Permissible torque varies according to the nature of the duty. One of the following torque definitions should best approximate to any application.

Peak torque:the maximum load torque sustainable for a minimum of 10<6>static torque reversal cycles at maximum compliance
Application: Periodic operation of valves, switches or other incrementally-operated loads.

Nominal torque:the maximum non-reversing load torque sustainable at maximum compliance.
Application: Unidirectional drives, typically pumps, fans and dynamometers.

Reversing torque:the maximum reversing load torque sustainable at maximum compliance.
Application: Predominantly inertial loads associated with position or velocity control. Typical direct loads: encoders, resolvers and tachogenerators. Typical reflected loads: slide tables, linear positioning systems.

Fixed and floating shafts

Fixed shafts:Couplers with radial flexibility are required when both shafts are conventionally located by two bearings.

Floating shafts:Floating shafts self-align and make a true angle with the adjoining shafts. Radial location is provided by the coupler. Only angular flexibility is required, or desirable, in the coupler.

Huco Engineering
• Tel: 01992 509888
• Fax: 01992 509890
• Contact: Ray Buttifant

For 78-year-old Howard Arneson, who invented the famous Arneson surface drive propulsion system, slicing through ocean swells at over 100mph is just a piece of cake.

The Arneson drive, now made under license by US-based transmission systems manufacturer Twin Disc, is probably the most popular propulsion system in high speed boating. It is used widely on racing boats, cruisers, patrol boats and yachts of all sizes. A variation of the drive is even used in military applications, including US Army tanks.

This is a surface drive that can be steered and trimmed in the water while the craft is moving. It runs with the propeller partly out of the water. This design reduces appendage drag (such as a dragging gearcase) and cavitation, which detrimentally affects efficient propeller performance.

The Arneson drive pivots port and starboard like a stern drive, working without a rudder. This pivoting controls the direction of propeller thrust, thus improving steering response. A boat with this drive can be trimmed while moving to get the right degree of submersion for the load and condition of the water.

With a surface drive like the Arneson, the propeller and drive train extend aft, not down in the water. This positioning reduces draft, along with the noise and vibration usually conducted from below through the hull. It also improves propeller bite. With the drive behind the transom, the designer of a high performance boat can place the engine as far aft as desired.

Finally, the hydraulic rams which are key to the Arneson drive are positioned outside the boat, freeing up space inside. Propeller depth is controlled by a vertical trim cylinder permitting 15deg of up/down motion. A horizontal steering cylinder allows 40deg of port-starboard trim.

Arneson has now reached 175mph piloting the 46ft Skater, a new generation catamaran powered by a 4500hp Lycoming gas turbine similar to that giving flight to a Chinook helicopter. The initial propeller shaft for the Skater was made by Ziegler Industries from 17Cr-4Ni precipitation hardening stainless steel, known for its good combination of high strength, high hardness and corrosion resistance. The finished shaft measures 40in long by 2.5in diameter in the centre, tapering down to 1.875in at both ends. The shaft is installed using a double cardon joint with a front-end drive, propeller at the other end and bearings at both ends.

But after 50 hours of running time at speeds of around 100mph, the shaft broke off, dropping to the bottom of the sea with its propeller. Up to then, the shaft had not been subjected to the shear forces of the severe acceleration or sustained, record-setting high speeds which Arneson anticipated. The President of Zieger – Don Zieger – suggested making the propeller shaft from Custom 465 stainless – a premium-melted, martensitic, age-hardenable alloy made by Carpenter Technology. This alloy can reach a 260ksi UTS when peak aged (H900 condition). In this condition, it has excellent notch tensile strength and fracture toughness.

When over-aged in the H1000 condition, Carpenter’s Custom 465 stainless provides a superior combination of strength, toughness and stress corrosion cracking resistance when compared with other high-strength PH stainless alloys.

Condition H1050 was selected from the heat treatment schedule to get a hardness of RC45-46 and maintain essential straightness. Test have shown successful performance of the shaft at speeds of 175mph, sustained for several hours.

• Carpenter Specialty Alloys

February 2000