Distance between axes of the input shaft and screw rotors in a screw compressor

[mechanicaljw]

Hello All,
I have a general question about whether the distance between the input shaft and the rotors of a screw compressor is important? The axis of the input shaft of our machine is parallel to the axes of the rotors of the lobe compressor and in between are the first stage and the second stage gears. I am thinking that if this distance is large the force that would be transferred to the rotors and hence the bending on them would be large?? But i cannot justify it.

I would love to know what your views are.

Thanks!
Jimmy

 

[JJPellin]

I am having a difficult time picturing what you are describing. Are you referring to the axial spacing across the drive coupling? Please provide additional detail about the machine train configuration.

Johnny Pellin

 

[mechanicaljw]

Hello Johnny,

A sketch of the drive train is attached. I am referring to the vertical distance (L) between the input shaft and the rotor shaft.

Thanks!
Jimmy

 

[JJPellin]

I don't see any relationship between the distance you specify and the loads on the compressor rotor. Since there are two sets of gears in between, the forces are a result of the gears. I assume that the gear is mounted directly on the end of the compressor shaft. The amount and type of force will depend on the gear configuration (single helical, double helical, straight spur), the speed and torque, the tooth design (profile and pitch), the gear ratio and the gear diameters. There are probably gear experts who can direct you to an appropriate reference to determine what these forces would be.

Most of the gears that I work with impose a couple of forces on the shafts that support the gears. There will be a separating force trying to drive the gears apart. And, there will be a force associated with the force line where the gear teeth contact to transmit the driving torque. These can be combined into a single resultant force for steady state conditions. If you want to analyze start-up and shut-down transient conditions, it will get more complex.

In general, if I wanted to reduce the resultant forces on the compressor rotor, I would move the gear inward (axially) as close to the compressor as possible and increase the diameter of both gears while maintaining the same gear ratio.


Johnny Pellin

 

[mechanicaljw]

Hello Johnny;

Thanks for the info. I appreciate it very much. I do have another question. It would be good if you can share any knowledge you might have on the choice of timing gears in screw compressors. In our current machine that we are having excessive noise in the timing gear is helical. Do you know whether using a spur gear would make any difference?

Thanks!
Jimmy

 

[Strong]

Spur gears are generally considered to produce more noise than helical gears. If trying to reduce noise, then first check for gear tooth wear, correct timing and bearing condition, and then test for natural frequencies on gears and case. Damping or mass-stiffness modifications can then be installed to reduce resonant vibrations and radiated noise.
Walt