The 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 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.
The design also minimises horizontal movement during this process.
Horizontal (alongside) drives will also use dual springs, when the mass of the motor is unsafe to use a single spring (pivot). These are referred to as H-series (HA) drives.
The late 1990s saw greater emphasis on pump and crusher drives up to 630kW. Fitting, adjusting or changing out vee-belts and/or pulleys had major safety issues – particularly with larger overhead drives. These were all mounted on conventional jacking screws.
Overhead drives are more likely to present safety issues than those at the ground level. In most cases, reaching up above chest height is considered “part of the job”. Increasing the force required by adding a length of pipe is also considered “part of the job”. All of these add to personal risk and injury. Common accidents include strain injuries, falls and bruising.
In addition, mechanical problems due to design and/or corrosion should not be overlooked, eg an inability to make parallel the drive and driven shafts, to face drive and driven pulley, and to achieve and maintain the correct drive belt tension as per the belt manufacturer’s specifications.
In today’s market, labour is the single largest cost and companies strive for efficiencies and cost reductions. So, why do some end users ignore cost-effective designs that can reduce labour costs?
Leverlink had a mining client that accepted as normal practice that it took a full eight hours, and four men plus a crane, to change out a set of vee-belts on a large pump drive. All of its pumps were conventionally mounted on jacking bolts.
Leverlink designed a Verti-lift (VL) motor base to fit the existing pump base frame, used the existing vee-belts, and recommended a new guard that could be removed by two men without the use of a crane. Windows were included in the guard at belt mid-span for checking belt tension or re-tensioning.
The initial design included details such as ensuring the drive and drive shafts were parallel and that the drive and driven pulleys were faced. Unless the motor was removed or perhaps new pulleys fitted they would remain so at each and every belt change-out.
The cost savings were undeniable. Two men in two hours now did what took four men and a crane eight hours. The payback period was well within the extractive producer’s financial model and it went on to install some 30 additional pump motor bases.
Motor base design life
The design life of Leverlink’s VL and HA series is 10 years. This estimate is based on the rubber cord service life used in the spring. Other factors that may be of influence include maintenance, environment and installation.
Motor bases can also be rebuilt and refurbished. This may extend their service life to 30 years or more. Many extractive sites recognise the cost benefits.