CONVEYING
THE MERITS OF RUBBER TORSION SPRING MOTOR BASES IN SCREENS
The rubber torsion spring motor base has a lengthy history. Richard Sharp explains how it came to be devised and why it is such a critical part of extractive and broader industrial processes.
T
he first rubber torsion spring motor base for vibrating screens was developed in Australia in the late 1980s. It was based on the rubber torsion spring design credited to Hermann J Neidhart in the 1940s. At that time, a rubber torsion spring unit was attached to a simple base plate. This was clamped to a predetermined angle, about that of the required torque for the functional specifications of the application. On vibrating screens, when connecting the drive vee-belts directly to the screen excitors, it results in the pulley to pulley centres not being fixed. That is, the pulley centres change when the screen starts and stops (during resonance) as the screen body is mounted on steel coil springs for isolation purposes. This results in drive belt slippage in the start-up phase, and high vee-belt and pulley wear will happen.
CATEGORY A AND B MOTOR BASES A “Category A” motor base with a pre-loaded rubber torsion spring allowed the drive pulley to follow the action of the screen in resonance, maintain sufficient belt tension to avoid belt slippage and reduce the force transmitted to the supporting structure. This helped to extend vee-belt life, as well as the drive and driven pulley life, and resulted in lighter motor support structures, as the mass of screen was not being applied during resonance. It should be noted that motor base designs fall into two distinct groups: • The “Category A” (Figure 1), which allows for the electric motor to be connected directly to the outer section of the rubber torsion spring. This allows for a resilient mounting and is ideal for applications such as vibrating screens or feeders when the vee-belts are connected directly to the excitor drive. The downside is the compression of the rubber cord under load allows misalignment of the drive and driven pulleys. To overcome this effect an alignment bearing is fitted to the drive side. This is referred to as a “dynamic application”. • The “Category B” (Figure 2), which has the electric motor connected to the inner section of the rubber torsion spring. In turn, this is
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Quarry May 2019
Figure 1. On a Category A motor base, the electric motor is connected directly to the outer section of the rubber torsion spring.
Figure 2. On a Category B motor base, the electric motor is connected to the inner section of the rubber torsion spring.
connected via bearings to the side plates of the motor base. The outer section of the spring is linked to a tensioning device for fitting and/ or changing drive belts and applying torque to maintain drive belt tension. This design is for static applications. That is where the drive and driven pulleys are fixed. By the early 1990s, most vibrating screen drives rarely exceeded 30kW. However, once the market demanded larger motor bases for bigger screens and feeders with drives requiring motors 37kW and larger, it was obvious an improved mechanical adjustment was required. Fully enclosed and lubricated mechanical tensioning devices were then developed.
These have the benefit of being able to purge old grease and lubricants at regular intervals (eg twice a year) and remain failure-free. These designs have lasted 20 years.
DUAL SPRING MOTOR BASES Category A motor bases are without exception single spring (single pivot) in their design. However, Category B motor bases (Figure 3) can be both single and dual spring arrangements. The criteria are safety and mechanical design. All large overhead motor base designs are dual spring and operate in a parallel configuration to lift the mass of the motor and tension the belts in a safe and efficient manner.
