Sizing Coupling Between Compressor and Driver

The coupling will typically be sized based on a torsional vibration analysis as much as on shaft size and horspower. For a motor there is also torsional issues if it is a synchronous motor, and starting torque issues if it is a induction motor.

Bottom line, this is a pretty engineering intensive process and should be undertaken only with the guidance of someone experienced. Your machinery and coupling OEM's should be consulted.

Dear Steve S

Firstly, I did compressor size. After I used starting torque of compressor performance, and then I got induction motor size. Which hp would you refer to? Compressor rated hp or motor hp? How would I know shaft size? If I know the shaft size , I can find at related coupling catalog. Can you give me one example.

Thanks and rgds,

Cherry

Hi Cherry,

Always use the highest calculated value before applying safety factor.

I am not entirely sure but I would have thought that certain compressor types would have been "grouped" by now and you should be able to "generalise" the vibration factor by looking at couplings that are correct for power and shaft size. The coupling types should "under ideal" give you an indication whether they are suitable for your application. I am sure however, that after you have done your calcs, the coupling manufacturer will be willing and able to help further.

I think you may be putting the cart before the horse as we say in Texas. Job one is to select a compressor based on the gas process conditions. Once you have that, the compressor OEM will likly also suggest both a drive and a coupling, but at least will give you a shaft end size.

If you are rerating a machine, then you should have the compressor shaft diameter and gas horspower, and so you can then select a drive and a coupling. Again, for a recip compressor it is important that a torsional vibration analysis be carried out as you may need a torsionally damped coupling for the application.

I highly recommend you send an e-mail to jo oak below and get his help.

Dear Steve S./Jo oak

Yes, based on gas process condition, I selected a compressor first. I got compressor rated kw 5816.5kw and calc 3196kw, 215.9mm shaft size, rated rpm 1200 and calc: rpm 710. According to start up speed torque curve of the compressor performance gear inertia 26.64 kg m2, friction breakaway torque 2847.22NM and mean torque 3407.6NM@710rpm, I selected the induction motor. For this motor there are two shaft dia 240mm and 190mm , power 3685 kw.

I don't know max: distance between shaft ends (DBSE), I guess the distance about 1m between reciprocating compressor and induction motor. Service factor should be 3.0. All shaft material would be stainless steel because of off shore application and gas gathering. In that case , am I able to know coupling size? but, I don't have torsional analysis. Can we guess coupling size without torsional analysis?

Thanks for your guidance.

Best regards,

Cherry

Why do you need to guess? Are you trying to do an initial layout of the train?

You need the torsional analysis to tell you both DBSE and coupling size. It is the torsional characteristic of the train that will tell you both the style and size of coupling necessary.

Again, this is an analysis usually done by the compressor OEM...

Dear Cherry,

One of the leading Highly Engineered Coupling in the Industry is John Crane International Ltd. (UK & USA)

To answer your questions, please note the following:

1. Essential informations for coupling selection

• Driver Machine
• Driven Machine
• Power (HP or kW ) Normal

• Max. Transient or Torque (Nm or lbs.ft) if available

• Speed (rpm) Normal

• Maximum if available

• Distance between shaft ends (DBSE) (mm or inches)
• Driving and Driven shaft types and diameters

2. In coupling design the basic stress analysis considers the steady-torque condition.

3. Select a suitable Service Factor

3.1 Steady Torque (eg. Centrifugal compressor), SF = 1

3.2 Cyclic Torque (eg. Reciprocating compressor), SF = 3

Load Classifications: SERVICE FACTOR

# Continuous & uniform operation (SF=1.0)

(Infrequent starts/Low starting torques)

# Continuous oeration w/ minor torque (SF=1.5)

fluctuations or frequent start/stop cycles

# Torque loading varies during operation in (SF=2.0)

conjunction w/ some shock loading

# Substantial torque fluctuation & heavy shock loads (SF=2.5)

4. Determine Coupling RATING:

Given: Power (kW)

Speed (RPM)

RATING = kW x 1000 x SF

N

Where: SF=Service Fasctor; kW=Power; N=Shaft Speed

5. Choose Coupling Arrangement

The coupling arrangement will depend upon the following:

# Available DBSE (distance between shaft ends)

# Bearing arrangement of the coupled machines

# Allowable overhung moment

# mating interfaces with the shafts

# special features required (if any)

If you need more info just email me, jojie.apolinario@johncrane.co.uk or jojie_oak@yahoo.com

Regards

Jojie