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Like all mechanical equipment, disc-stack centrifuge users encounter mechanical issues with their machines. Some of these problems are simple to fix and don’t have any long-term effects on the centrifuge. On the other hand, some of the mechanical issues mentioned below can cause severe damage to the centrifuge if not timely rectified.
The following is a list of typical causes of excessive vibration levels in a disc-stack centrifuge. We explain each reason separately with a suitable remedy to alleviate the vibration. We have a detailed article about disc-stack centrifuge bowl vibration in our blogs.
A disc centrifuge vibrates primarily due to the bowl being out of balance. Uneven sludge distribution and incorrect assembly are causes of an unbalanced bowl. At the operating speed of over 5,000 RPM, this unbalance leads to the centrifuge bowl’s excessive vibration and, therefore, the centrifuge itself.
The following table lists the leading causes of bowl imbalance with individual fixes.
– Bowl Parts Misaligned: Disassemble centrifuge bowl and check assembly. It is quite common to see misaligned interior bowl parts before bowl tightening.
– Not Enough Bowl Discs: Count the number of bowl discs per Parts Catalog. An incorrect number of discs leads to a loose bowl assembly that causes vibration.
– Mismatched Bowl Parts: Check the bowl serial number of matching parts. There are multiple centrifuges of the same type, and accidental interchange of bowl parts between the centrifuge in locations where there are numerous centrifuges occurs. Since each bowl assembly part is individually balanced, these mismatched parts cause the bowl to vibrate.
If the sludge is sticky or clumps up, the sludge can build up within the bowl unevenly. This non-uniform sludge distribution causes an imbalance in the bowl leading to vibrations.
The operator needs to open and clean the bowl to get the bowl back in balance.
The top chamber of the top disc houses the oil-paring disc. The incorrect height of the paring disc causes the top disc to contact the paring disc. This contact can be intermittent, which leads to the vibration of the entire bowl assembly.
Dismantling the bowl and checking the paring disc height shown in the operation manual is the corrective action needed to remedy this cause of vibration.
The bowl assembly of the disc-stack centrifuge is mounted on the vertical spindle. Any deviation from the vertical axis of the spindle can cause severe vibration of the bowl. The repair manual shows the proper way to check the spindle runout.
If the spindle wobble exceeds the specified tolerance, it is necessary to replace the spindle to fix this cause of vibration.
Precision, high-speed bearings facilitate the bowl's rotation at operating RPM with a heavy, dynamic load. Foreign metallic impurities in the lubricating oil and general wear cause bearings to lose their efficiency over time. A worn-out bearing causes the spindle to move laterally, which leads to significant vibration at operating speed.
All bearings of the disc-stack centrifuge should be replaced as part of the centrifuge’s major overhaul periodically to avoid vibration caused due to bearing wear.
Disc-stack centrifuges are mounted on the base frame through vibration-isolating dampeners. These dampers are also known as rubber cushions. The elastomer of these rubber discs loses its elasticity over time and rigidize.
Rigid dampeners cannot absorb the centrifuge’s inherent vibration, amplifying through the base, and the entire centrifuge experiences excessive vibration.
Replacement of centrifuge vibration dampers is essential to eliminate this potential cause of centrifuge vibration.
The vertical bowl spindle is supported by the spring casing, which houses radial springs to absorb the spindle vibration. If one of these springs breaks due to material defect or wear, the spring casing cannot absorb the vibration of the spindle, leading to the bowl and centrifuge vibrating.
Checking spring support springs during normal maintenance and in case of excessive vibrations can help resolve this issue.
An unusual smell from a disc-stack centrifuge is typically a burning or charring smell. Often this smell is routine. However, frequent smells and associated vibration or other indicators are worth investigating quickly.
The majority of older-style disc centrifuges use a friction clutch for torque transmission. The friction between the clutch pads and the steel drum is at high speed, and that can cause some smell. This smell is a regular occurrence on startup and dissipates once the centrifuge is up to speed.
The friction brake slows down the bowl during the shutdown process gradually. However, it is vital to release the brake once the centrifuge has come to a stop. Often the operator forgets to release the brake. On startup, the brake pad is pushing against the transmission drum. The ensuing friction causes the smell.
Therefore it is essential to release the brake once the centrifuge has come to a stop. Also, it is a good idea to check the brake state before starting the centrifuge.
The gear oil refers to the lubricating oil in the disc centrifuge transmission housing in the lower frame. It is crucial to maintain the oil level in this housing to the prescribed level.
Low gear oil leads to the transmission components heating up and the bearing losing the splash lubrication essential for their operation. Heated parts in the transmission can cause the lubricant to burn and create an unusual smell.
The operator must maintain proper lubrication oil in the gearbox housing.
Other disc-centrifuge articles of interest......
Disc Centrifuge Backpressure - Comprehensive Guide
9 Steps to Selecting & Buying the Right Industrial Centrifuge
Centrifuge RCF and RPM | Difference & RCF Calculation
Disadvantages of a Disc-Stack Centrifuge | Illustrated Guide
Difference Between Decanter & Disc Centrifuge | Technical Comparison
All centrifuges generate a certain amount of noise while operating. However, increased operating noise from a centrifuge indicates a possible mechanical fault or impending failure. The following are some of the leading causes of excessive noise in a disc stack centrifuge.
As explained above, low gear oil causes excessive noise in addition to the smell. The loss of friction between the gears causes the engaging gears to cause unusual noise.
The paring disc pump is a centripetal pump within the disc-stack centrifuge bowl, which converts the fluid’s rotational energy to pressure. This part resides in the top chamber of the top disc. This chamber has close tolerances, and therefore, the height adjustment of the paring disc is critical.
If the height of the paring disc is not set correctly, the stationary paring disc makes contact with the rotating top disc. This contact causes a noise indicating the touching of the two parts.
The paring disc height must be checked and immediately corrected to remedy this issue. This correction will eliminate the noise associated with the contact.
A disc centrifuge has main transmission gears housed in the bottom gearbox housing.
The big brass gear on the horizontal shaft mates with steel gear on the vertical spindle. Regular operation leads to wear on the brass gear being softer. Excessive wear leads to teeth mismatch, which leads to unusual noise from the gearbox.
Inspection of the gear set and replacement of the complete set is the remedy to fix this cause of the noise.
Any worn-out or faulty bearing will cause unusual noise from the centrifuge. As explained in the previous section, replacing the bearings is the solution to this issue.
The coupling pulley transmits the torque to the horizontal shaft through the elastic rubber plate. There is a specified clearance of 2 millimeters between the two surfaces.
If the coupling pulley is mounted incorrectly on the motor shaft and presses against the elastic plate, it pushes the entire assembly. This force causes a lateral load on the horizontal shaft and the bearings supporting it. This undesired loading can cause noise.
In case of unusual noise from the horizontal shaft location, check the clearance between the coupling pulley and the elastic plate. Adjust the coupling pulley to maintain the specified gap.
All disc-stack centrifuges have a specified operating speed. If the centrifuge does not reach its design speed within a specified time (centrifuge specific), the cause of the low operating speed needs to be investigated and corrected.
The operator can monitor the bowl speed by checking the Revolution Counter’s rotation on the front cover in the front of the centrifuge. See the photo below, showing the revolution counter.
As expected, if the brake is engaged, the friction between the brake pad and the transmission drum puts a drag on the spindle’s rotation preventing the centrifuge from coming up to speed.
Releasing the brake is the solution for this cause of slow bowl speed.
Sometimes, the gearbox’s oil can seep over to the clutch side through a leaking oil seal between the two compartments. This oil can coat the friction pads reducing their ability to transmit the torque to the horizontal shaft.
Worn-out friction pads also prevent the friction clutch from transmitting the torque to the drum.
A quick inspection of the friction pads is needed to ascertain this condition. The replacement of friction pads is the obvious solution for both scenarios.
A leaking bowl (as explained in the “Bowl leaking troubleshooting guide”) causes the fluid to eject from the bowl through the sludge ports. The escaping fluid causes friction with the air in the bowl cavity. This obstruction causes resistance to the free rotation of the bowl, which reduces the bowl's speed.
The first indication of a leaking bowl is higher than average motor current and fluid flow through the sludge outlet. In either case, stopping the centrifuge and checking the bowl seals for the cause of leakage is the solution to this problem.
A motor defect causes the motor to generate insufficient torque, which causes the bowl not to get to operational speed.
A thorough check of the motor and motor cables will confirm this as the centrifuge slow-down cause. The replacement of the centrifuge motor is the remedy for this problem.
Again, bad bearings directly lead to abnormal friction, which leads to low operating speed.
A periodic check of bearings and replacement as needed is the only solution to many mechanical centrifuge issues.
Disc-stack centrifuges for different current frequencies have different gear sets. The difference is that the 50 Hz motor operates at 1,500 RPM while the 60 Hz motor runs at 1,800 RPM. Given that the centrifuge bowl needs to rotate at basically the same speed, the transmission gears’ design ensures that the bowl spindle rotates at the same RPM, irrespective of the voltage frequency.
Now, if a set of gears designed for 60Hz voltage frequency is accidentally installed and operated with 50 Hz voltage, the gear ratio will prevent the bowl from coming up to the designed speed. Checking the gear set and replacing it with the correct set of gears fixes this cause of low bowl speed.
Following the above description, the centrifuges for 50 Hz and 60 Hz operation also have friction blocks of different weights. The weight difference between the two friction blocks transmits the same torque via friction at a lower (50 Hz) speed as at a higher speed (60 Hz). The 60 Hz blocks are lighter than the 50 Hz blocks.
In case a centrifuge is operating at 50 Hz voltage and is fitted with 60 Hz blocks (lightweight), the torque transmitted will not be sufficient to get the bowl up to the designed speed.
Wrong friction blocks can cause low bowl speed and need checking as part of an insufficient bowl speed investigation.
The presence of the friction clutch in most centrifuges causes a surge of current during startup. This current drops down and stabilizes once the centrifuge bowl reached operating speed.
When the startup current is above average values, it is crucial to investigate its high current cause.
The following are the common mechanical causes of high startup current in disc-stack centrifuges.
As explained in the previous section, the operator may inadvertently install incorrect friction blocks in the centrifuge. If the operator installs a set of friction blocks designed for 50 Hz operation in a 60 Hz centrifuge, the extra weight of the blocks will exert higher torque on the transmission. This additional load on the motor will cause the motor to draw a high current on startup.
Checking and replacing the friction blocks (in case of wrong ones are installed) is the fix to this cause of high startup current.
All disc centrifuges have a designated rotation direction, as indicated by an arrow on the centrifuge frame and motor. The clutches rotating in the wrong direction cause incorrect contact, which leads to excessive friction and high current.
As a regular practice during maintenance, the motor direction of rotation should be checked to avoid this cause of high startup current.
If the brake is engaged, it puts a drag on the horizontal shaft. This drag is acting against the motor, which leads to a high current during startup.
Again, releasing the brake fixes this cause of high current. It is good practice to check the brake handle position before every startup.
As mentioned above, each centrifuge has a specific range of startup current depending on the voltage and motor size. Under certain conditions, this current may be lower than usual. The following are possible causes of low startup current.
Most of the causes and remedies of low startup current are similar to those of low bowl speed.
Following the discussion above regarding incorrect friction blocks causing low bowl speed, in the same situation, the motor current also will be lower than usual because the friction blocks are not able to transmit the design torque. Thus incorrect friction blocks (lightweight) lead to low startup current.
As in the low bowl speed citation, it is using the correct friction blocks is the fix to this condition.
Oily and worn-out friction pads slip and are unable to transmit the torque. Similar to being the cause of low bowl speed, they are also often responsible for low starting currents.
The replacement of friction pads is the solution to this issue as well.
A defective, faulty, or worn-out motor cannot produce enough power to get the centrifuge to operating speed due to fiction block slippage. This slippage causes the motor to draw less than the normal amperes.
If the motor is drawing less than the usual current, the motor should be rectified and replaced as necessary to fix this cause of the low starting current in the centrifuge.
Every disc-stack has a pre-determined startup time based on the centrifuge size and motor. Over time, the operator may notice the startup time getting longer. There are a few causes of longer startup time, most of which are related to the motor’s torque transmission to the bowl.
The above section, “Low Operating Speed,” describes the causes and remedies for each issue. The following is a list of reasons for a long starting time.
The stoppage time is the duration between the brake’s application and the centrifuge bowl coming to a complete stop. Long stoppage time is usually due to brake-related issues. The main is similar to the one listed above related to the friction pads slipping.
The brake system uses a brake pad to slow down the rotating drum gradually. However, if the brake pad gets coated with oil or is worn, the brake becomes ineffective.
Like friction pads, the brake pad’s replacement is the solution to fixing the long retardation or stoppage time.
The transmission housing is the chamber in the bottom half of the disc centrifuge, which houses the gear set. The oil in this housing is the primary lubrication for the centrifuge. Any contamination of this lubricant is detrimental to the centrifuge transmission and bearings.
Draining a little bit of the transmission oil periodically is always a good idea to check for water accumulation. Water settles at the bottom of the oil and drains out first. Any presence of water in the drained fluid should be investigated and rectified.
The following are the leading causes and individual fixes for water in the transmission housing.
The casing drain in the main outlet for the operating water to exit the centrifuge bowl cavity. An accidental plugin or closing of this drain causes the water to accumulate inside the bowl cavity. This water can then seep into the transmission housing below, contaminating the oil.
The spring casing supports the upper bearing on the bowl spindle. There is a seal under the spring casing which isolates the bottom transmission housing from the upper chamber. If this seal is damaged or not fitted during reassembly, water from the operating water device can enter the bottom transmission housing.
Replacing this upper bearing seal is necessary if water is present in the transmission housing. The operator should fill fresh, clean oil in the transmission housing after replacing the bearing seal.
Another source of water in the transmission housing is condensation. Water condenses inside the housing, especially in humid climates. This water can accumulate over time and damages the gears and bearings.
On rare occasions, the operator might notice oil under the centrifuge base. It is the first sign of oil leaking from the gearbox or transmission housing. The following are the three leading causes of oil leakage and relevant fixes for each.
The centrifuge bottom bushing holds the bottom bearing. There is a seal or gasket under the bottom bushing, which keeps the oil from leaking. Incorrect installation of this seal or gasket is the leading cause of oil leaking from the centrifuge gearbox.
The removal and inspection of the bottom bushing is the only way to identify this issue, and replacement of the seal or gasket is the relevant fix.
An external locknut holds the bottom bushing in place within the centrifuge frame in specific disc-stack centrifuges. This nut can loosen over time due to the centrifuge vibration, and a loose nut allows the oil to leak out from under the bushing.
A loose locknut also allows the bottom bushing to rotate along with the rotating spindle. This bushing rotation can cause severe damage to the bushing and the frame leading to major repairs.
Each disc centrifuge requires a specified volume of lubricating oil that needs to be in the gearbox. A sight-glass on the bottom frame indicates the level of oil. The oil level in the transmission should be to the mid-level of the sight glass.
If the operator inadvertently fills too much oil into the transmission, the excess lubricant can leak out through the cover plate on the front of the bottom frame. To fix this leakage, the operator should drain some of the oil.
The operator should also check the cover plate gasket for damage, which can also cause the oil to leak out.
The above-mentioned mechanical issues related to disc-stack centrifuges commonly occur, and timely remediation of these issues ensures the long-term reliability of these industrial centrifuges. Most of the fixes mentioned in this article are quite simple and can be part of routine maintenance.
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge