Smooth Operators: Wichita multi-plate clutch technology

Smooth Operators: Wichita multi-plate clutch technology smooths out damaging torque spikes during grinding mill start-up

On geared ball and SAG mills with large inertia loads, multi-plate clutches are often utilized to provide smooth torque ramp-up to the mill’s pinion gear speed while keeping the power spike down on the motor current draw as the mill starts to rotate.

After mill-to-motor speed synchronization, multi-plate clutches can also provide a level of protection during torque overload conditions in the mill by slipping if an unforeseen load spike occurs. This slippage protects the mill’s pinion gear, ring gear and gearbox (if installed), which are all expensive items with typically long lead times.

Some mills install drum style clutch units. However, multi-plate clutches have greater swept areas for the metal center plates as compared to drum clutches, allowing better heat absorption. Another disadvantage of drum clutches is that when they spin up to speed, their torque value can drop off due to centrifugal force.

Multi-plate friction disc clutches utilize a very compact shape factor to provide a high level of torque. In grinding mill applications, compact multi-plate clutches can be sized based on diameter and clutch assembly length, a real advantage compared to drum style units.

Smaller-sized mill operators sometimes use fluid couplings and a mechanical lock-up feature for smooth startups. But these types of solutions are horsepower-limited in their ability.

The clutches are used on geared mills only because of the position of the mill drive motor. Gearless motor configurations don’t require a clutch because the motor is built into the frame of the mill.

Multi-plate, shaft-to-shaft clutches are typically installed as close to the pinion shaft as possible to provide the greatest level of protection of the driveline components during an overload condition. The two main driveline configurations are:

>> Low-speed motor > clutch > pinion
>> High-speed motor > gear reducer > clutch > pinion

Due to their respective functionality, ball and SAG mills each have different torque overload requirements. For example, the torque overload capacity requirement for a ball mill is 200%. A SAG mill requirement might be 225-250% torque based on Full Load Motor Torque (FLMT). This is the amount of torque the drive motor can produce on a temporary basis during start up.

Wichita Clutch has designed its multi-plate Grinding Mill Clutch (GMC) specifically for use on challenging ball and SAG mill applications.

As previously mentioned, heat dissipation is very important during and after a mill start up from a stationary position. This is when the most energy is created during the slipping of the clutch to bring the mill speed up to match the motor speed. High levels of heat are also generated during mill jogging and inching operations.

All this energy is dissipated in two areas, the friction material and the thermal mass of the mating metal rotors or center plates.

The friction material-to-center plate interface area is where this energy is created during the clutch slipping at start-up. Temperatures can be as high as 600°F and the slip time can run as long as 10 seconds, but normally 3-6 seconds. The thermal energy is stored in the metal center plates as heat rises in these plates.

Wichita Clutch has developed a unique high-coefficient of friction material specially-suited for GMC applications. Its blending of semi-metallic, organic and high temperature phenolic resin binders makes it the ideal choice for GMC units. The material is heat fade-resistant and is made to provide long friction disc life if used within normal limits. In service for many years, this robust friction material installed in GMC units continues to provide customers with a solution that can withstand heavy-duty mill startups on a repetitive basis.

The GMC incorporates an annular-shaped air tube which is constructed from reinforced neoprene rubber in a vulcanized fabrication. This makes for an easily changed unitized part as compared to a metal piston and O-ring seals. It also provides a very responsive way to create force to compress the friction discs against the metal rotors or center plates to transmit torque from the motor, through the clutch, to the mill. This force creation is very repeatable and has an extremely low hysteresis as compared to a piston-style actuator.

The air control system requirement for a GMC unit installed on single pinion drive mill is fairly basic and can be accommodated with standard-size airtube pressure ports. However, dual-pinion drives require a more robust air control system with much larger air tube pressure ports to provide a very responsive torque ramp-up for the mill drive.

Most larger GMC models are shipped with the optional Quick-Change Adapter that features a set of parts that allows the drive ring, on the outside of the clutch, to be more easily moved back away from the clutch. This allows access for friction disc replacement without having to disassemble the clutch completely. This development was driven by growing operator desire to reduce clutch maintenance man-hours associated with disc replacement in the field.

Wichita also offers friction discs that are split into two 180-degree segments to assist in helping customers replace worn friction discs. This eliminates having to completely disassemble the clutch unit for disc replacements, saving many man-hours of work.

Wichita GMC units offer more torque in a smaller package for a less expensive overall install and long life if properly used and maintained.