Questions & AnswersCategory: GeneralSULLAIR 1150/350 TWO STAGE DRILL COMPRESSOR
stephen zeveska asked 1 year ago

hello!  i’m attempting my first screw compressor re-bearing job and i need some technical info.  i have asked these questions on numerous places and nobody wants to give up the goods!! lol!!

1  i need to know rotor to end-plate clearances on both low and high stages

2  rotor bearing preload on both low and high stages

3  input shaft bearing preload

any help with this would be GREATLY appreciated!!!

1 Answers
Cas Staff answered 1 year ago

Hi Stephen,

Unfortunately, I have to disappoint you as well. Those are highly guarded secrets of the compressor manufacturers.

There are many independent companies that do screw element overhauls, and they basically found out about these clearances by measuring them on good elements or by trial and error.

Too tight clearance basically limits the max temperature the element can run at, because it will seize quicker. A too high big clearance will decrease the efficiency/performance of the element.

I can only give you some general steps and general values for overhauling screw elements.

  1. Remove pulley/gear on drive side
  2. Remove gear cover and pull out bearings
  3. Open up complete element from the other end (discharge side) – you can pull out the two screws together with the end-plate as one assembly.
  4. Measure the clearance between the screws and the end-plate as reference. (with feeler gauges).
  5. Take out the bearings and remove the screws from the end-plate
  6. All bearings should be replaced at the same time
  7. Clean and inspect the screws themselves – you can use sanding paper and Scotch-bright to clean the screw surface and remove any damages/high points
  8. Re-assemble.

You don’t need many special tools. Torch, pulley pullers, bearing heater, etc. You’ll probably need to make a tool to keep the screw from turning when tightening/loosening the bolts on the screw shaft end-plates (to stop rotation, don’t use the other screw – it will damage the screw profile).

For the clearances, I can only give you some general, unofficial, no-warranty values:

  • Air inlet side: 0.25 – 0.5 mm
  • Air outlet side: 0.08 to 0.15 mm

(these are for single-stage oil-injected rotary screw compressors)

As you can see the outlet side is the important one – bigger clearances here mean more compressed air can ‘escape’ and the elements performance will decrease. A too tight clearance means that the element will seize when it heats up.

Hope this helps 🙂

Let me know how it works out. Good luck with the job.

And if you don’t mind, I would love to get some pictures of the air-end rebuilt for use on my website – much appreciated if you want to share!

Thanks and all the best,