To crown or not to crown a lagged pulley
Pulley crowning is an essential design element for tracking control of flat conveyor and power transmission belts. A friction cover, or “rubber lagging” helps to increase the traction between the conveyor belt and the drive pulley. Used alone, these covers optimize belt performance. However, when combined with a crowned pulley, there can be undesirable performance effects such as joint degradation and premature belt wear. As a result, Habasit generally recommends avoiding this combination.
To explain why problems arise, we need to understand the interaction between the conveyor belt and a lagged, crowned pulley. Let’s start by looking at the fundamentals of how the crown works to keep a belt centered on the pulley.
Crowned pulley design
In order to produce a centering influence on the belt, a crowned pulley is designed with end diameters that are smaller than the pulley’s central diameter. This creates tapered surfaces on which the belt moves, which push its edges towards the center of the pulley. The different diameters also produce different circumferences around the pulley.
Under tension, the belt conforms to the crown profile, with the applied tension force greatest in the belt’s central area. This corresponds to the pulley’s largest diameter/circumference. During operation, the belt and pulley move together in this area of greatest contact force. As the pulley’s diameter/circumference gradually decreases toward its ends, the length of the belt path on the pulley and the contact force decrease proportionately. Although the belt path on the pulley’s tapered ends changes, the edges of the belt at these ends must continue to move with the belt center. As a result, the edges of the belt slip against the tapered ends of the pulley, and they do so more where the difference in belt path length is greatest. The decreasing contact force in these areas helps to facilitate the slip. It is this slip between the belt and the tapered ends of the pulley that generates the friction reaction and resulting forces that push the belt’s edges toward its center.
Let’s look now at what can happen when a pulley is both crowned and lagged.
When a lagged pulley is crowned, the friction forces increase – and this is when the potential for problems begins. If there is sufficient friction force from the lagging, the greater centering effect can cause the belt to buckle in the center and rise up off the surface of the pulley. Ultimately, the belt can buckle completely and fold over.
The stick-slip effect
With increased friction between the belt and the crowned roller, the resistance of the outer belt edges to slip increases proportionately. For example, if belt friction increases from 0.15 on a steel pulley surface to 0.35 against lagging, the resistance to slip increases by ~133%. This means that as resistance to belt slip at the outer belt edges increases, the edges will have a greater propensity to drag behind the center of the belt and produce lateral deflection (deformation) of the belt.
There can also be damage to the lagging as a result of the stick-slip effect, or “chattering”. This occurs when the outer belt edges resist slip due to static friction hold, but then break the friction hold and suddenly spring forward, pulled by the center area of the belt. As the belt slips, it rubs forcefully against the surface of the lagging material, producing abrasions in the form of flat spots on the lagging. In more extreme cases, under high-stress belt operation, the belt joint may deform to the point of failure. The vulcanized lagging material can also experience reversion, and become soft and sticky, leaving rubber residue on the conveyor belt, slider bed and/or rollers.
Belt construction and tension play a key role in this effect, as they help to determine the size, severity, and number of abrasions. For example, a laterally rigid belt will not deform as much as a more laterally flexible belt, and will spring back sooner, along a shorter distance, while developing smaller, more numerous chatter marks. On the other hand, a belt under high tension will create greater friction force against the lagging, causing it to hold fast for longer and finally slip with greater abrasive force, leaving fewer but larger chatter marks.
Conveyor belt construction
What’s the best way to deal with these problems if a design situation requires a crowned and lagged pulley? One answer is to think very carefully about the belt construction and specify a highly durable rubber, such as carboxylated nitrile rubber (XNBR), or crosslinked polyurethane with a brushed or ground finish, in the 80-90 Shore A hardness range. This offers less friction than the commonly employed 70 Shore A, and thus minimizes abrasion.
Understanding the principles of the function of the pulley crown and the dynamics between the belt and the crowned pulley can help you to optimize your belt and conveyor performance, which in turn can increase your belt maintenance intervals and extend belt service life. If you have any further questions, download our Engineering Guide or contact us!