Centrifuges

A centrifuge is a simple device based on the principle that rotating a liquid separates particles and liquids of various densities. From this simple principle myriad devices have been designed.

The simplest centrifuges have a single-speed motor, a mechanical timer, and a rotor that holds the samples at a fixed angle of between 20° and 55°. For user safety, the lid of the centrifuge should have an interlock on it so that the unit will not spin with the lid in the up position.

A simple rotor is made from a piece of metal with angled holes for the samples drilled into it. Balancing the rotor is very important; if the user has only a few samples to be spun down “dummy tubes” may be necessary to properly balance the load. Since the rotor is attached to the motor shaft, any unbalanced load can cause motor damage and uneven speeds.

Other types of rotors have sample carries—buckets—holding numerous samples that are vertical at rest but when spun swing out to a 30° to 90° angle as set by mechanical stops on the carrier.

Rotor breaks can cause injury to those working in the area, and regular rotor inspections should include looking for cracks in the metal. Particular attention has to be paid to centrifuges that have rotors that can be changed out. If the knob securing the rotor to the motor shaft is not fully tightened, severe damage to the device and laboratory can occur when the rotor breaks loose while spinning.

There are several other areas that can cause problems:

• speed not calibrated
• timer out of specification
• brushes wear
• door interlock jams
• not correctly balanced

Rotation Speed
The simplest centrifuges have a fixed speed ranging from 2,500 rpm to 10,000 rpm. Some units may have two speeds that are switch selectable.

The simplest variable-speed centrifuges will have a rheostat speed control, which may be nonlinear. A common practice with variable-speed units is to mark one or more points on the speed indicator with a measured speed. Most of the newer variable speed centrifuges do have built in tachometers that provide the users with a speed indication. If there is a built-in tachometer, it should be checked for accuracy at incoming and at each preventive maintenance inspection. The measured speed should be accurate to within ±5% of the indicated speed.

Other speed-control systems can involve SCRs, stepper motors, and servo systems. They must be verified on a regular basis to assure accurate results. On larger units and ultra-high-speed centrifuges, drive belts between the motor and rotor are not uncommon. Drive belts have to be inspected on a regular basis to assure both accuracy and user safety.

Low speed centrifuges have spin rates up to 12,000 rpm, high-speed units go up to 35,000 rpm, and the ultra-high speed units can reach 125,000 rpm.

Samples in high-speed and all ultra-high -speed units are refrigerated because friction caused by the air will dry them out and change results. As part of the normal PM inspections, the accuracy of the temperature indicator is checked. Temperature measured should be ±5°C. Typically temperature is 4°C for refrigerated samples.

Timer
Centrifuges have a built-in timer that is either electronic or mechanical. Depending upon the centrifuge, the time can be set from seconds to minutes. A considerable number have a “hold” feature that allows the unit to run for an indefinite amount of time. Also the centrifuge may have a device on the start that delays the spin for several seconds after the rpm rate and timer are set and the start button pushed.

Mechanical timers break and need lubrication from time to time. If possible, the timer should be lubricated during a PM inspection. Unless the manufacturer has specified particular performance requirements, the accuracy of the time should also be checked, and anything more than ±5 seconds at the 60-second setting needs to be corrected. For times between 1 and 59 minutes use ±10 seconds.

Electronic timers are more accurate, but need to be checked during the PM inspection. Times less than 60 seconds should be accurate to within ±3 seconds; from 1 to 59 minutes, ±10 seconds; and for times over 60 minutes, ±30 seconds.

Brushes
Most low-speed (less than 10,000 rpm) centrifuges use carbon brushes to make electrical contact with the rotating part of the motor. These brushes wear down over time and need to be replaced. They should only be replaced with brushes of the same size. Do not use undersized brushes as they may wear unevenly and score the shaft of the motor. Brushes are held against the shaft via spring pressure; and, if the spring weakens, breaks, or is missing, the motor may not spin. If the caps holding the brushes in place become loose or cracked, the brushes can lose contact, and the motor will not run at all or will not run consistently. High-speed centrifuges tend to use induction or capacitors to achieve faster speeds with low maintenance. Ultra-high-speed units usually have three-phase motors, which, if equipped with brushes, will have three instead of the common one-phase, two-brush type.

Brushes that are installed properly with the correct tension wear evenly and have a bright, almost shiny look on the contact end. If the brushes are defective or not making good contact, the contact face of the brush will be dull and rough.

Braking System
If the rotor were to be left to stop on its own, it could take a long time for it to drop from 100,000 rpm to stop. To cut the time, most units are designed with a brake. This is not a mechanical device, as on a car, but a switch that reverses the electrical field in the motor to get the motor spinning in the opposite direction. The operator has to energize the switch and should only hold the switch in the “reverse,” “stop” position for a few seconds at a time. In some very old, low-speed centrifuges there may be a mechanical brake that is applied by pushing a lever. These are rare.

Cover Interlock
All centrifuges produced after 1990 are required to have an interlock system that does not allow the rotor to spin unless the cover is closed. Some of these interlock systems are very simple: A solenoid that pushes a rod through a hole in the cover latch is common. Others are more complicated and may involve several solenoids, flexible cables, and a clock. The clock can be tied to the rpm indicator so that the solenoids will not be released until a set time has elapsed after the speed drops to zero. These “timed” units may give the appearance of failure because the operator cannot immediately open the lid. Check the manual to confirm both the delay and if that delay is adjustable.

Gaskets
Damaged gaskets can affect the centrifuge by altering its speed, changing the balance of the rotor, and altering how the interlocking system works. Gaskets should be carefully inspected during each PM cycle and replaced as needed. Gaskets will also dry out and particles may flake off, affecting the samples being spun.

Feet/Casters
The feet on benchtop units are often like suction cups that help anchor the centrifuge to the benchtop. They may have to be adjusted so that the centrifuge is level and remains stationary while in use. As they age, the feet become less effective and might have to be replaced. They should be checked on the PM cycle.

Floor units, in most cases, will have casters with brakes on at least two wheels. They may also have leveling feet that have to be adjusted. This adjustment may lift one or more casters off the floor, which makes movement to work on the unit difficult.

David Harrington, PhD, is director of staff development and training at Technology in Medicine in Holliston, Mass.

Contributing to this article is Trever Tiller, a biomed with the Technology in Medicine program in Virginia.