Repair of screws

Can anyone tell me of any experience they have with strip down of screws for on site repair. I know its standard practice to change the bearings after 12000 hrs of operation. Does anyone do this or strip the rotors out or capacity control vane on site or is it too specialised and requires removal for workshop inspection.

Hi, Reggie, we do all our overhalls on site. Most work including thrust bearings and checking of the slide/loading cylinder can be done without removal of the compressor from the base. For rotor removal and main bearing work the compressor requires removal from the base. As for bearing replacement I would be inclined to beleive 12K Hrs is small Hrs for replacement of thrust bearings. All screws have what I call a pre-loaded bearing assembly, ie you fit the bearings and load them up by torqueing the bearing covers. You would generally replace the slide loading piston seal yearly and at this stage (6K Hrs) you would check the tolerences of the thrust bearings. This involves fitting a clock gauge to measure the horizontal movement of the compressor shafts when the tension is let off the preloaded thrust bearings. Main bearing on the compressors are slightly different, dependant on whether they are roller bearing as in the thrust bearing or plain metal bushes. The main bearings are generally replaced on Hrs run at anything from 30K Hrs to 60K Hrs.
All these proceedures will be discussed in the relevent compressor workshop manual, you will probabely be able to check tolerences without much technical assistance, but if they require changing you are better with some help from someone who has done a similar compressor before.
Regards. Andy.

Andy thanks for your advice much appreciated. Where would i obtain manuals for overhauls for a good read?? I take it that you use them. Ive tried in the past to get a hold of them but they seem to be like rocking horse ****.

Hi, Reggie, the easiest way you can obtain this information is from a web site, try the following.
www.bitzer.de
This site has information on axial clearances in a screw.
www.grass.nl
I haven't checked about downloads, but I know they have a disc with maintenance manuals on it.
Regards. Andy.

Hi Reggie, Depending on who's screw compressor you are using or which O.E.M. packaged the unit will determine how much information is available. For some reason certain manufacturers think that they have the worlds biggest secret. In the Southeast United States most end-user's are getting away from rebuilding their own compressors as well are not using a contractor to do the work for them either. Most O.E.M. equipment manufacturers offer rebuild/exchange programs that carry a one (1) or two (2) year warranty. If a company calculates the cost of parts, tools and man-hours and the fact that there is no warranty it makes sense to use the respective manufacturers programs for rebuilds.
If you look at an older version of an operators manual and compare it with the newer version for the same model machine you will notice the recommended time to go internal to a screw has changed drastically.
With the aid of a good planned maintenance program with periodic vibration anaylisis they pretty much run the compressors without internal maintenance until vibration anaysis determines its time to swap out compressors. We found that this saves the customers money and downtime in the long run as well limits the liability for the end user and contractor and burdens the manufacturer only.

I tend to agree with jryffel on this at least here in the states. Some compressors you could maybe do without too much trouble in the field. Some I would avoid altogether. Add specialized tools, manuals, equipment, labor and the cheap cost of factory remanufactured units makes a better job. In some parts of the world though it is common practice to rebuild screws and from talking with some of the fellas doing it they have quite good results. I'm not sure if they are doing it on a bench or what. Probably it may be cheaper to do that considering the location and cost of shipping a whole compressor and shipping the core back or availability. Andy sounds familiar with rebuilds on screws and I would also tend to agree that 12K is short for a bearing change. More like 50K + with vibration analysis. Some of the smaller hermetics units I work with are pushing 40K without any internal work, just yearly oil analysis and no vibration analysis.
Larger open drives, definetly do vibration and oil analysis on a regular basis.
Mike Hopkins

With SAB163's, it costs us around £6500 for a service exchange unit from Sabroe that comes with a full warranty. This is providing there is no major internal damage that requires the casing to be replaced.

We change between 30000 to 40000 hours depending on vibration testing. (I am wary of oil analysis on its own as I had discharge end bearings start to go that vibe analysis picked up but the oil analysis showed no problem).

So £6500 spread over 35000 hours is not too bad a deal. And we can change a screw within in a normal working day :)



Steve