There comes a time in an old pump’s life when it develops problems of low pressure.

Is your syringe pump suffering from these symptoms?

  • Low pressure – your pump doesn’t deliver anything like the pressure the manufacturer claims
  • Doesn’t stop pumping when the syringe pusher block hits the end stop. Could be a stalling problem too.

drivenutbuttonThere Might Be a Quick (and Cheap) Fix

There is a thing called the Drive Nut Button on your pump.

When you press this button it lets you move the pusher block freely up and down the lead screw – the long threaded rod shown in this image. It’s used when you are loading syringes.

This lets you adjust the position of your pusher block to where your loaded syringe plunger handle will be.

When you release the drive nut button, it re-engages the drive nut on the lead screw.

It is very easy to release the Drive Nut Button and not have it properly engage the hidden nut and the worm screw. I call this condition ‘riding the worm’.

If your drive nut is ‘riding the worm’ you will not get full pressure from your pump.

If your pump is allowed to stall often, slowly but surely your drive nut will wear and need replacing.

At the same time your stall detection mechanism will not trigger, and your pump will not automatically stop pumping when the pusher block bottoms out on the collar or the end block. This will effect both the infuse and withdraw directions. It might damage expensive small glass syringes too.

‘Riding the worm’ has it’s own special sound – that of a plastic clicking where as failed stall detection has a rubbery repeated thumping sound as the teeth of the rubber drive belt slip on the motor drive – slowly eating your belt.

Riding the worm effectively means an end to unattended pumping because you can’t trust the pump to stop.

How to fix it.

If you pick up your pump and try to set the drive nut button such that your ‘riding the worm’ you will convince yourself it’s almost impossible to do and if you did do it, it would self correct as soon as the pump started moving.

When it happens to me, I firmly grasp the pusher block and rock it from side to side until I hear a clear and fairly loud ‘click’. This is the drive nut engaging properly.

Don’t let your pumping program stop by waiting for a stall – set a target volume and stop the pump before a stall occurs.
If it’s happening all the time, your drive nut needs replacing – contact your pump supplier.


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