TechArchive

Original article date: April 1995

Material choice, sizing, pulley design and geometry should all be taken into account in selecting the right pulley design.

When designing a pulley for a metal belt application, the most fundamental consideration is establishing whether it will be used as a friction or timing pulley.

A timing pulley used with a metal must be designed with an o.d. such that the metal timing belt will be driven at its neutral axis. With metal belts, the o.d. is known as a Tape Support Diameter (TSD). This ensures smooth engagement and disengagement of the belt to the pulley. For most systems, the neutral axis is one-half the belt thickness.

High throughput, precision indexing for automated assembly and inspection systems require low weight and low rotational inertia pulleys. When the system index profile is integrated to the belt/pulley system, solid round stock pulleys may not be ideal. Alternate pulley body designs are the I-Beam and the Capped Tube. The I-Beam design is used for pulleys up to 200mm of face width, after which the Capped Tube becomes the preferred design. Both systems are relatively expensive, as I-Beam requires a great deal of machining and Capped Tube requires increased fabrication.

A critical design factor is the diameter of the pulley. There are two considerations:

• the diameter which is best for any design constraints in the system
• the diameter which best manages bending stress in the metal belt to ensure optimum performance and longevity.

The proper pulley diameter is defined in terms of a pulley diameter to belt thickness ratio. Ideally, this ration should be 625:1 or more. This relationship typically results in total stresses which are one third of the yield strength of the metal belt (with total stresses defined as the sum of bending, working and loading stresses).

As the diameter to thickness ratio decreases, belt bending stress increases, and belt life is reduced. Based on bending stress life testing, Table 1 details expected belt life in a friction drive system.

How is the metal belt going to be tracked? Traditional elastomeric belt tracking designs, such as crowning the face of the pulley or using flanges, are only used selectively to assist in tracking metal belts. Crowned pulleys help to stabilise any off-tracking characteristic of a metal belt, but will not self-centre the metal belt. Flanges, with the exception of PTFE types, are used at the expense of belt life.

The tracking characteristic of a belt is a function of uncontrolled stresses and forces acting on it. The belts and pulleys are tuned relative to one another using pulley axis adjustment. The belt is steered to a stable tracking state, with the centre of the belt width at the centre of the pulley width.

Traditionally, axis adjustment has been introduced by adjusting pulley shaft terminating pillow blocks left/right and up/down, with belt tracking accomplished by an iterative process of such adjustments. Using an independently steerable pulley is substantially easier – the design is based on the use of a steering collar and bearing assembly which press-fits into the body of an idler pulley. Rotating the pulley shaft rotates the steering collar, which changes the face angle of the pulley relative to the shaft. This controlled use of stresses and forces tracks the belt quickly and dynamically.

Metal flanges are not recommended, as the rotary scraping action against the belt edge results in greatly reduced belt life. A recently developed design using glass-filled PTFE works with metal belts as thin as 0.127mm. PTFE flanges are attached to the pulley body via a bolt hole circle in both the flanges and the pulley body. While a viable design option, prudence dictates that this design should be used as a short-term forced tracking influence.

Expected belt life in friction drive systems for different pulley diameter to belt thickness ratios

• Use large diameter pulleys relative to the belt thickness

• Use friction drive, rather than timing drive whenever possible

• Make the idler pulley adjustable for belt tracking/steering purposes

• Minimise the number of timing elements engaging a belt

• Crowned and/or flanged pulleys are not universally recommended for metal belt applications

• The precision and repeatability of a metal belt is only as good as the precision of its complementary pulleys.

Belt Technologies has published its new design guide and engineers’ reference for pulleys. This guide complements and expands upon design information incorporated in the sister publication on metal belts, which was published in 1995.

• Belt Technologies
• Tel: 0191 383 1830
• Fax: 0191 383 1920
• Contact: David T Huntley (Managing Director)

April 1995