LEARNING AND DEVELOPMENT
‘Low-speed centrifuges have maximum rotor speeds of less than 10,000 rpm, which do not require the rotors to be run in a vacuum’
instruments have an electrical braking system that functions by reversing the polarity of the electrical current to the motor. Other machines may have a mechanical brake.
Centrifuge tubes It is advisable to use a conical-bottomed tube in a swing-out bucket rotor for the sedimentation of cells. This tube type will retain the pellet of cells more effectively as the supernatant is removed. All tubes for use with high-speed rotors are round- bottomed. Pyrex glass tubes can withstand forces of around 2000 xg, while Corex tubes can be used up to 12,000 xg. Polycarbonate or polyallomer are the most common plastic tubes in use but great care must be taken when using organic solvents. Manufacturers usually provide extensive information about solvent, salt and pH resistance, as well as sterilisation procedures.
High-speed centrifuges are capable of rotor speeds up to 21,000 rpm, although super-speed instruments are capable of rotor speeds of 30,000 rpm, in which RCFs of 120,000 xg are possible.
Preventive maintenance • If the bearings on the upper and lower ends of the motor shaft are not of the sealed type then they should be lubricated as per the manufacturer’s instructions.
• Brushes should be removed regularly and checked for wear; they should be replaced if they are worn to more than one-half of their original length. When reinserting used brushes, replace them in the same orientation. New brushes should be broken in by slowly accelerating the unloaded unit to mid-speed and then allowing it to run for a period of time.
• The rotor, buckets and shields or carriers should be examined for signs of mechanical stress (cracks, corrosion).
• Some manufacturers etch the expiry date on the rotor and this should be checked periodically.
• Regularly lubricate the contact areas between the centrifuge buckets and the pins.
• Regularly check the condition of the O-ring on the tie-down nut on top of the rotor, and replace it if worn or damaged.
• Always follow a manufacturer’s specific instructions.
THE BIOMEDICAL SCIENTIST MAY 2016
241
National Cancer Institute/Diane A Reid
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