Disc Centrifuge Lubrication: Oils & Intervals | Dolphin

The disc centrifuge lubrication system uses an oil splash type method to lubricate the bearings. The large bull gear is partly submerged in the lube oil sump. The constant rotation of this gear splashes the oil into the bearings on the horizontal and vertical shafts. This splashing oil lubricates and cools the bearings.

A typical disc centrifuge has three main lubrication zones, each with different requirements:

Function: Lubricates the worm gear or helical gear drive, the lower spindle bearing, and (on some models) provides cooling for the gear drive assembly. This is the highest-volume lubrication point on the machine.

Oil type: The manufacturer recommends the type of lube oil for disc centrifuges. Alfa Laval recommends a viscosity grade VG 220 (ISO 3448/3104) with a viscosity index VI greater than 92 (ISO 2909). It is imperative for the user to refer to the manufacturer's recommendations for the correct oil type, as different manufacturers recommend different oils based on their centrifuge design and operating conditions.

Oil quantity: The volume of lubricating oil required depends on the size of the centrifuge. Most disc centrifuges feature a sight glass on the bottom frame housing to show the level of lubricating oil in the gearbox. The oil level should be halfway in this sight glass after the lube oil replacement.

Change interval: The lubricating oil in a disc stack centrifuge should be changed every 2,000 operating hours or every 12 months, whichever is earlier. In a new installation or after the change of gear transmission, change the oil after 200 operating hours and clean the gear housing. When the centrifuge is operated for short periods, lubricating oil must be changed every 12 months, even if the total number of operating hours is less than stated above. Larger centrifuges or belt-driven centrifuges have different oil change intervals — consult the manufacturer or operating manual for the correct frequency.

Checking oil level procedure:

1. Stop the machine and allow the oil to settle for at least 5 minutes.
2. Wipe the sight glass clean and read the oil level.
3. If below the lower mark, add oil through the fill plug slowly.
4. Never overfill above the upper mark — excess oil causes churning, foaming, and elevated operating temperature.

Function: The top bearing supports the upper end of the spindle against lateral forces and maintains spindle concentricity at operating speed. It is the most highly loaded bearing on the machine and the one most likely to fail first if lubrication is neglected.

Lubrication type: Top bearings are lubricated in one of three ways depending on machine design:

• Oil bath: A separate small oil sump around the top bearing. Requires separate oil level monitoring and periodic oil changes.
• Forced oil circulation: Oil is pumped from the main sump to the top bearing. Correct oil flow rate is essential — check the oil supply line for blockage if bearing temperature is elevated.
• Grease: Some smaller models use a sealed grease-packed top bearing. These require regreasing at specified intervals using only the specified grease type and quantity.

Oil type (oil bath models): Typically a light turbine oil or spindle oil (ISO VG 32 or ISO VG 46). Do not use gear oil in the top bearing sump — the higher viscosity increases churning resistance and generates heat.

Change interval: On oil bath models, change the top bearing oil every 1,000–2,000 hours or every 6 months. Inspect for contamination (cloudiness, metal particles, process fluid ingress) at every oil level check.

Grease type (grease-lubricated models): Use only the specified high-speed bearing grease. High-speed bearings require a grease with the correct base oil viscosity and worked penetration for the application — a standard lithium grease designed for slow-speed bearings will degrade rapidly in a high-speed centrifuge bearing.

Function: Electric motor bearings support the motor shaft and absorb both the motor's own rotating mass and the drive belt tension (in belt-driven models). Motor bearing failure causes motor damage, shaft misalignment, and vibration that propagates into the centrifuge drive system.

Grease type: Motor bearings require the grease type specified by the motor manufacturer, not the centrifuge manufacturer. Most standard IEC or NEMA motors specify a lithium-complex grease with an NLGI grade 2 consistency. Do not mix grease types — incompatible thickener combinations can cause grease softening and loss of film strength.

Regreasing interval: Motor bearing regreasing is typically required every 2,000–4,000 hours depending on motor speed (rpm) and bearing size. Higher-speed motors require more frequent greasing. Refer to the motor nameplate data or manufacturer guide for the exact interval and grease quantity. Overgreasing a motor bearing causes overheating — the grease purge hole must be opened before regreasing if equipped.

Access: Most centrifuge motors have grease nipples (Zerk fittings) accessible through the motor terminal box cover or through dedicated access plugs. Some totally enclosed motors are sealed-for-life and require no grease maintenance.

The following are the primary reasons to change the oil in the disc centrifuge lubrication system.

Metal Particle Contamination

The constant meshing of the transmission gears of the disc centrifuge causes wear and tear on the gear teeth. This wear leads to metal particles getting into the lube oil and the bearings through the lubrication. Lube oil with metal particles is the primary cause of bearing failure, which can have catastrophic effects on the centrifuge. The replacement of the old oil with new, clean oil prolongs bearing life and prevents unanticipated centrifuge breakdowns related to bearing failure.

Water Contamination

Water contamination of centrifuge lube oil is a common occurrence. The most common source of moisture contamination is condensation. In humid climates, condensation accumulates in the gear housing and contaminates the oil. Operating or process water can also seep into the transmission housing due to defective seals or O-rings. This type of water contamination is often quite severe due to the quality of water that can get into the oil due to a seal leak. Lubricating oil containing water causes bearing failure because water does not have the lubricating properties needed to operate the bearing at high speeds, as in a disc centrifuge.

Oil Deterioration

Lube oil loses its lubrication properties over time due to chemical decomposition and temperature variations. Reliable bearing service requires good lubrication. Therefore, periodic replacement of lube oil in the centrifuge is essential for the reliable operation of the bearings.

Alfa Laval recommends the following grade of lubricant for its centrifuges:

The following table lists the recommended oil from the different lubricant manufacturers:

The volume of lubricating oil required for each centrifuge depends on the size of the centrifuge. Larger centrifuges have a larger gearbox holding capacity and therefore need more lubricating oil. The following table lists the volume of oil required for some popular disc stack centrifuges:

There are specific field lessons that can help the centrifuge operator monitor and maintain the oil in the centrifuge transmission. The following is a popular tip based on these lessons learned.

Oil Quality Check and Water Drain

Using this tip, the operator can quickly check the quality of the lubricating oil and drain any accumulated water simultaneously.

During the centrifuge shut-down period, slowly unscrew the oil drain plug. Do not remove the plug altogether. Unscrew the plug until a small amount of oil starts to drain out. Collect this oil in a small container.

If the drained oil sample shows lots of water and or metal particles, it is vital to change it. This method allows the operator to drain small amounts of water if necessary.

It is essential to replenish the removed oil with fresh oil to the original level in the oil sight glass.