Overhung Load (OHL) in Opposing Directions

[kyphorrhinos]

Been reading posts on this site for a while and I finally decided to join so I could ask a question myself. Anyway...

My question is regarding Overhung Loads on shafts. Specifically, what happens when there are opposing loads? For example, I have a setup that will drive a pump and a generator from the same pulley. Four belts will go to the pump and four to the generator. They will be almost exactly 180 degrees apart. See image below.

Do the opposing loads produce a net OHL? The calculation from most manufactures does not seem to take this into account. Below is an example of a formula:

Sideload => L = (HP * F * SF) / (rpm * D)
where L is sideload, D is diameter of sheave, SF is service factor, and F is load factor.​

Based on this formula, the 450 HP @ 1800 rpm engine will need a unit with an overhung load capacity of almost 12000 lbs. Too me, it seems there will end up being a net load on the bearings, therefore, the capacity should be less.

Any ideas on how to calculate this net OHL?

Thanks in advance.

 

[Compositepro]

Adding the second load will tend to reduce the net over hung load on the driver shaft. You need to draw a free body diagram and add all the force vectors (belt tension and direction) to get the net force. OHL also depends on how far out on the motor shaft your pulley is from the motor bearing. With a short shaft this number is fairly constant.

 

[kyphorrhinos]

Seems to easy. The OHL does not return the same values as calculating the force exerted by the belts. Do you think using a FBD with the OHL will produce correct results?

 

[gruntguru]

Yes. If the FBD is done properly.

je suis charlie

 

[chicopee]

You'll have an OHL from the driver weight plus a side net load horizontally to the right and to the left due to driven gears #1 and #2. These vectors added will have a somewhat downward resultant either to the left or to the right of the center of the driver. In addition you have to take into consideration the torques from friction at bearings and the startup inertias of the three gears, motor, generator and pump in order to ensure that the HP is adequate.

 

[MikeHalloran]

You also have to cover the case where either load is declutched, i.e., you have to cover the belt tensions individually. The 'resultant' is unimportant.

Mike Halloran
Pembroke Pines, FL, USA