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In-depth: Vee-Belt drive systems on vibrating screens

Darren Toth, director of ToThink Engineering, explains the benefits of vee-belt drive systems on vibrating screens and some common (and avoidable) issues associated with them.

Although there are many different methods of driving machines, vee-belt drive systems are quite common on many different types of equipment in quarries and recycling fixed plants. More specifically, on vibrating screens, the vee-belt setup is particularly common for several reasons.

Vee-belt drives are efficient and compact for the amount of power they are capable of transmitting. When it comes to vibrating screens this compactness, combined with the need for an offset between motor and screen shaft, makes cat-walk level the ideal place mount the motor.

Vee-belts are smooth and quiet, and slip is negligible. They cope very well in dusty environments because there is nowhere for the dust to collect (or become trapped) and cause problems. While vee-belt drives are not suitable for extremely high speed or very low speed applications, vibrating screens appear to be within an ideal speed range for this type of drive system. Vee-belt drives are also a relatively low-cost arrangement.

The drive motor on some vibrating screens is mounted on a pivoting motor base. That is, a base that allows movement of the motor but with a mechanical resistance provided by some sort of flexible/spring device. This spring is typically a rotating rubber-element or a steel coil spring. The benefit of using a pivoting motor base it that vee belt tension can be maintained over a much greater range of movement of the screen shaft (relative to the motor). This significantly reduces the peak tensions in the belts (particularly during start up and shut down) and also helps to keep the belt seated in the pulley groove if/when the belts become slack.

While it isn’t always necessary to install a pivoting motor base on a screen, a properly set up base will significantly increase the life of the vee belts as well as other components such as motor output shaft bearings and pulleys. A motor base will also greatly reduce the chances of throwing a belt on shutdown. Such an event can lead to significant damage to the belts and the need for their replacement.

Poorly designed, installed and/or maintained systems can be quite problematic. Here are a couple of details to be aware of that may help avoid making some common mistakes.

Many vibrating screens with vee-belt drives are supplied, by the original equipment manufacturer (OEM), with deep groove pulleys (both the screen pulley and the motor pulley). That is, the grooves are deeper than those of a standard specification pulley and so the vee-belts sit well inside the major (outer) diameter of the pulley. This greatly reduces the chances of throwing a belt as there needs to be a lot more movement before a belt can be slack enough to flick off.

We have had customers who have had their non-OEM suppliers replace pulleys based on the vee-belt size and the major (outer) diameter of the pulley but assumed the pulley was a standard pulley. When the worn deep-groove pulley was unknowingly replaced by a standard groove pulley, not only was it far more likely to ‘throw’ a belt (as the grooves were no longer deep) but the pitch circle diameter of the pulley was significantly altered and, as a result, the operating speed of the screen changed. In the case of changing a motor pulley, this error would lead to an increase in rotational speed of the screen. It would have the opposite effect on the speed if the same mistake was made with the screen pulley. There can be risks associated with changing the speed of a screen and that is a conversation for another day.

A well-designed vibrating, circular (or oval) motion screen (with a vee-belt drive) is typically supplied (by the OEM) with an eccentricity (radial offset) in the screen pulley. That is, the centre of the outer rim of the pulley is not in the same location as the centre of the shaft. The purpose of this is to negate (or cancel) the motion of the screen in the direction along the line between centre of motor pulley and centre of screen pulley. In simple terms, the intent is to minimise or eliminate any tugging action (on the vee-belts) caused by the vibration (throw) of the screen. The direction of this offset, relative to the screen counterweights, is critical. The message here is to be aware of this because, again, we have had customers who have been supplied replacement pulleys without this offset and it has caused premature wear or failure of drive components.

Now on to pivoting motor bases. When incorporated into a vee-belt drive system, there are some simple guidelines that if actioned help maximise the effectiveness and service life of the motor base.

Firstly, wherever possible, orientate and position the motor base so that the weight of the motor contributes to the belt tension rather than reducing it. That is, set up the base with the motor mounting plate offset from the centre of the pivot of the base and mount the base and motor so that the motor is on the opposite side of the pivot from the screen pulley. In setting the motor base up this way, the amount of tensioning required by loading up the rubber rotating element (or spring) is minimised due to the weight of the motor adding to the belt tension.

Secondly, aim to achieve as close to a 90-degree angle as practicable (see diagram) between the line joining the pulley centres and the line between the motor shaft and the motor base pivot. This means that any rotation at the pivot, caused by movement of the screen pulley is minimised. As a result, the peak stresses in the rubber rotating element (or coil spring) are reduced and the service life of the motor base is improved.

For some readers this information may help explain past (or current) challenges. The intent here is to raise awareness of some lesser-known details and to highlight the importance of sourcing the correct components from companies with the appropriate technical knowhow. •

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