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If you have an Alfa Laval disc stack centrifuge, it is crucial to know how to maintain the friction clutch to avoid centrifuge breakdowns and unplanned maintenance downtime. Also, a worn-out clutch causes a host of other issues for the centrifuge in the long run.
This article discusses the maintenance and repair of the friction clutch in an Alfa Laval disc centrifuge.
The friction clutch transmits the power from the drive motor to the centrifuge’s internal transmission. The centrifuge bowl is a relatively heavy mass that rotates within the centrifuge frame.
Given the momentum of such a contracted mass, it is necessary to increase the bowl’s speed gradually. The friction clutch functions as a transmitter of the torque gradually through friction between the friction pads rotating at the motor speed and the transmission pulley that rotates the bowl through a set of gears.
As shown in the illustration, the friction hub connects to the motor shaft. It rotates at the same speed as the motor. The friction blocks pivot radially outward due to the centrifugal force caused by the rotation.
The friction blocks have replaceable pads made of friction material mounted on them. These pads contact the inner surface of the transmission drum that is stationary. The centrifugal force pushes the pads on the drum surface, and the rotating pads transmit the torque to the drum through friction.
The transmission drum starts to rotate, rotating the bowl assembly along with it slowly. Over time, the transmission drum and the centrifuge bowl start rotating at the same speed as that of the motor.
It takes a centrifuge with a friction clutch between 2 and 10 minutes to come up to operating speed. A smaller centrifuge with a lighter bowl reaches operating speed quicker than a larger centrifuge with a heavier bowl.
Quick Tip: If the friction pads and drum surface are glossy/shiny, use sandpaper to roughen up the surface to get them working again!
An experienced centrifuge technician can typically inspect and replace the friction pads in the centrifuge friction clutch in about 30 minutes.
However, if the technician observes excessive wear on the drum or the friction blocks, the repair or replacement of these parts can take much longer.
A friction clutch has some crucial benefits for the disc centrifuge. Some of these are listed below.
The friction clutch brings the centrifuge bowl up to speed over a period of time, gradually. This gradual acceleration allows for the use of a smaller motor to power the centrifuge.
Though the friction clutch also has an initial current spike, the overall current draw with a friction centrifuge is spread over a more extended period of startup time and is lower.
The centrifugal friction clutch needs a simple DOL motor starter that is inexpensive, easy to control, and readily available.
The friction clutch does not have a direct, rigid connection to the centrifuge transmission shaft. Therefore, in case of sudden, unexpected loads on the motor due to centrifuge malfunction, the friction clutch prevents the load from reaching the motor. This safety mechanism avoids damaging the motor and electrical components related to the motor.
The friction clutch has its own set of limitations that prevent its widespread use in centrifuges. The following is a list of some of the critical limitations.
As the name implies, the centrifugal clutch uses friction to transmit the torque to the centrifuge. Fraction inherently causes wear on sacrificial parts. These parts, therefore, need a periodic replacement that adds to the maintenance of the centrifuge. The friction clutch also adds to the potential of unexpected centrifuge breakdown, as mentioned below.
Following the above-mentioned periodic maintenance, a set of friction clutch-related replacement parts needs to be stocked by the user. The stocking of these parts adds to spare parts inventory cost and storage.
The friction clutch transmits the motor-generated torque to the centrifuge horizontal shaft via the fraction between the pads and the drum. This friction has limitations on the amount of torque transmitted. Therefore, a centrifuge fitted with a friction clutch takes longer to reach operating speed than a centrifuge with direct coupling.
When the centrifuge is shut down, the drive motor is de-energized. However, due to the momentum of the rotating bowl and the absence of any other stopping mechanism, the centrifuge takes a long time to stop rotating.
The mechanical brake has a limited effect on stopping the centrifuge bowl quickly.
The friction between the friction clutch pads the transmission drum causes heat. Though the friction between the two parts reduces once the centrifuge is at operating speed, the friction clutch is still a cause of heat generation.
In rare cases, if the friction pads wear out and direct contact between the steel friction blocks and the steel transmission drum, there is the risk of generating sparks. The presence of sparks in any machinery is highly undesirable, primarily when operating in hazardous conditions.
Often, centrifuge operators ignore the friction clutch assembly until the clutch assembly malfunctions or breaks. If the friction clutch is not maintained correctly, it causes a lot of operational problems. Some of these problems are listed below.
The friction pads wear out over time due to the constant contact between the friction elements and the drum. If the pads have excessive wear, they cannot transmit the torque from the motor to the centrifuge transmission.
This inability to transfer the torque leads to the slower ramp-up of the centrifuge bowl speed leading to longer startup time.
Worn out friction elements or friction pads with oil on the contact surface leads to insufficient torque transmission. This loss in transmission can prevent the centrifuge from reaching optimum operating speed.
Low operating speed leads to multiple issues, from the inefficient separation of fluids to the failure of the centrifuge bowl to open and close for sludge discharge.
Friction pads attach to steel friction blocks that carry them and provide the weight behind the pads to push them against the drum. If the friction pads wear out completely, the steel friction blocks contact the steel drum.
Direct contact between the steel friction block and steel drum is highly detrimental to the centrifuge transmission. This contact will cause the blocks and drum to wear. Both of these components are cost-intensive. The replacement of these parts also involves centrifuge disassembly and re-assembly, a long and labor-intensive process.
As with all rotating parts in a centrifuge, the clutch assembly also has a balanced design. This design positions the friction blocks at equal angular increments to avoid vibration during rotation.
Uneven wear of friction pads causes an imbalance of weight of the friction clutch assembly. This imbalance causes the disc centrifuge to vibrate, and depending on the extent of friction pad wear, and the vibrations can be excessive.
The operator should watch a few telltale signs that indicate the friction clutch needs inspection and possibly maintenance. The leading indicators of friction clutch issues are listed below.
If the centrifuge takes a long time to reach operating speed, it will most likely be time to replace the friction pads.
If the centrifuge does not reach operating speed even after a prolonged startup time, the operator must inspect the friction clutch.
A high pitch screeching sound indicated a metal-to-metal contact between the friction blocks (pads are worn out) and the drum.
A broken or dislodged friction pad causes an imbalance in the rotating clutch leading to the entire centrifuge vibration. Any unusual centrifuge vibration could indicate worn friction pads or a missing/broken friction pad.
The maintenance of the friction clutch system is a quick and straightforward process. The process involves the following four steps:
The first step is removing the brake plate by extracting the four bolts holding the brake plate to the centrifuge bottom frame.
The small, L-shaped bracket is bolted to the rotating nave mounted on the motor shaft. This bracket holds the friction block on the swing pivot pin.
After removing the holding (L) bracket, the friction block should slide off the pin and is extracted from the centrifuge.
The operator then inspects the friction pads mounted on the friction blocks for excessive wear. It is also necessary to ensure all the brass screws holding the friction block are present.
The new friction pads come with new installation screws. Once the pad installation is complete, the friction block is refitted into the centrifuge nave.
You can order the friction pads and blocks through our Alfa Laval parts inquiry page.
It is advisable to inspect the friction clutch every 600 operating hours to check the condition of the friction pads. If the brass screws holding the friction pad show any wear at all, it is critical to replace the friction pads at that point.
The centrifuge user can choose to replace the friction clutch assembly with a direct drive coupler unit. This coupling eliminates the entire friction torque transmission mechanism and directly couples the drive motor to the centrifuge transmission shaft.
However, with the clutch, the entire mass of the centrifuge bowl is directly connected to the motor and cannot be brought up to speed instantaneously.
Therefore, a VFD (variable frequency drive) replaces the motor starter in the centrifuge control panel for this arrangement to work. The VFD gradually ramps up the motor to operating speed over a predetermined ramp-up time. Thus the VFD avoids excessive load on the motor during startup.
Yes, we routinely convert friction-driven centrifuges to direct-drive transmission centrifuges. We also modify the centrifuge controls to replace the motor starter with a suitable VFD during this process. The centrifuge drive motor also needs up-gradation to an inverter duty motor.
This article is based on our 40 years of disc centrifuge experience. To know more about us, visit the About Us page.