TechArchive

This article was originally written in the period 1995-2000

Do you know the difference between absolute accuracy and non-accumulation of pitch error?

Metal belts – thumbnail profile

• Durability:-a variety of alloys may be used, each with its own resistance to chemicals, humidity and corrosion. Engineers generally select a belt material on physical properties (belts have a high strength-to-weight ratio), availability and cost.
• No lubrication:-unlike the links of a chain, a metal belt is a single element and does not generate any component friction that requires lubrication. This reduces system maintenance, improves reliability and keeps the system clean.
• Non-stretchable:-spring steels with a high modulus of elasticity make metal belts virtually non-stretchable, compared to other belt types and chain. This helps in high performance applications for precision positioning.
• Smooth operation:-metal belts are free from the pulsation of chordal action often seen in other belt types and chain. This results in precise translation of the control system motion profile.
• Accurate and repeatable:-metal belts can be fabricated with a pitch accuracy of +/-0.013mm station to station, important in designing indexing, positioning or processing equipment.
• No static build up:-metal belts discharge static electricity, crucial in the manufacture of electronic components, such as integrated circuits and surface mount devices. Metal belts are also thermally and electrically conductive.
• Clean and clean room compatible:-unlike HTD or flat neoprene belts, metal belts do not generate particulates and are suitable for food and pharmaceutical processing. Metal belts do not require lubricants and will not generate dust that would introduce foreign substances into clean room environments. Additionally, they may be sterilised in an autoclave.

Plain metal belts are created by electron beam welding together two ends of a metal tape to form an endless belt. They are used in conveying, heat sealing, casting and imaging. Perforated belts are used in timing, carriage positioning, conveying and indexing.

Perforated metal belts can also be fitted with attachments and can then be used to provide positional accuracy and repeatability, to act as a product transport device or to control stages of a manufacturing process. Applications include precision position indexing for automated assembly, lead frame drives, timed transfer lines and packaging systems.

Metal drive tapes are made of the same metal strip as metal belts, but are not endless – they are fitted with end attachments or perforations. They can perform with zero or near-zero backlash in applications including carriage positioning, plotters, robot arms and optical element drives.

Often, attachments or pockets may be utilised to locate components while a vacuum drawn through a belt’s perforation is employed to secure the component in place during transport. Specific edge geometries may be developed to conform to component profiles, while through attachments locate the components and satisfy timing requirements.

All metal belts and drive tapes travel around pulleys, most of which take one of three forms: round stock, I-beam and capped tube. Pulleys generally serve one of two purposes: friction driving or timing. Surface treatments may be applied to one or both surfaces of a belt, tape or pulley. These include PTFE (Teflon), urethane, neoprene, silicone, anodising. Unusual surface treatments have included copper compounds, gold plating and powdered diamond bonding.

One of the most important advantages of a metal belt is its overall accuracy. Perforated belts or belts with attachments can be fabricated with pitch accuracies of +/-0.013mm. Unfortunately, absolute accuracy of timing belts is often perceived as being synonymous with non-accumulation of pitch error. But in designing an effective timing/indexing system, all that typically is necessary is the ability to repeat an operation precisely, again and again.

Repeatability is the ability to duplicate a set of conditions within a specific tolerance. Design engineers have often dwelt on the concept of accumulation of error when in fact repeatability should typically be the focus of attention.

A metal timing belt has a timing element (a perforation or anchor) engaging a complementary pulley timing element (a tooth or pocket). The belt timing element is specified with a dimension known as pitch or pitch length. Pitch has a tolerance, typically +/-0.013mm and this tolerance will accumulate through the length of the belt. Such accumulation can cause the design engineer two concerns, though both are arguably unfounded:

• that the belt will not position correctly through its length;
• that mis-registration of the belt and pulley occur as belt accumulation of error compounds relative to the pulley with each revolution of the belt.

In a two-pulley timing/indexing system, work is usually performed in the centre to centre distance between pulleys, with pitches moving on and off the centre distance as the belt is indexed. This movement of pitches can be thought of as a moving average of pitches relative to the total number of pitches in the belt length. Therefore what has become important is how constant – or repeatable – is the accumulation of error over the pulley centre distance.

The total accumulation of error in the centre to centre distance between pulley determines how other system components are set up in relation to the belt, such as feed systems and pick-and-place actuators. Once this setup is complete, the belt must hit these targets within some tolerance – the belt must be repeatable. While it may seem desirable to specify a pitch tolerance as being non-accumulative, it is not always necessary to achieve it.

Belt tracking

Given that a metal belt will not significantly stretch under tension, tracking a metal belt can be more difficult than tracking other types. A metal belt will not stretch to compensate for: lack of system squareness or alignment; uncontrolled pulley shaft deflection; differential loading; and belt camber.

Of these, the least familiar may be belt camber. Otherwise known as edge bow, this is the deviation of a belt edge from a straight line. Every belt has some camber. When placed in a squared two-pulley system and tensioned, one edge of the belt will be tensioned more than the other, because it has a shorter edge circumference. When the belt is rotated, this causes the belt to track away from the tight edge of tension, towards the loose edge.

The primary objective of any tracking technique is to counteract the influence of accumulative negative tracking stresses and forces (previously defined as system squareness, uncontrolled shaft deflection, differential loading and belt camber) with controlled stresses and forces, thus tuning the belt to run on the system.

Three basic techniques are used to track belts on systems using friction pulleys, timing pulleys or both:

• Pulley axis adjustment:-adjusting the pulley axis in a metal belt system is the most effective way of tracking a metal belt. Belt edge tensions are changed in a controlled manner, thus steering the belt. The technique is applicable to both flat-faced and crowned pulleys.
  Ideally, both the drive and idler pulleys would have adjustable axes. In reality, only the idler is adjusted.
• Crowning friction drive pulleys:-When crowned friction drive pulleys must be used, it is in conjunction with – not in place of – axis adjustment, because crowned pulleys will not self-centre a metal belt. Crowned pulleys work best on thin belts, as the belt web must conform to the crowned face of the pulley. While increased tension can be used to achieve conformity between the faces of the belt and pulley, tension cannot be so high as to cause permanent belt deformation.
• Forced tracking:-in cases where simple axis adjustment cannot completely eliminate improper tracking, forced tracking methods such as flanged pulleys and cam followers may be necessary and acceptable. System design relationships may need to change, such as using a thicker belt than might otherwise be recommended, since forced tracking techniques can contribute to a decrease in expected belt drive.

One particularly effective forced tracking technique employs a V belt bonded to the inner circumference of the metal belt. This two-element belt, called Metrak, distributes tracking stresses on the V belt rather than on the metal belt, maximising the belt life.

• Belt Technologies Europe
• Tel: 0191 383 1830
• Fax: 0191 383 1820