Drive train effieciency/power loss 1

[smitty1322]

I have noticed a common quesion in this forum lately on the effeciency of a gear train. I have a question to add to this. I am designing a drive for a hospital bed. I have my motor at the performance I need (motor only). I am using a transaxle, much like the electric scooters have, and I am wondering what that will do to my eff? My motor is 70% eff. at the .9 HP and output speed that I need.
Once I put this on my transaxle, my eff. will decrease.

My question is this, will the drop in eff. affect my torque or my speed? For example, say i had a 100% eff motor. Once I put it on any gearbox my eff dropped to 75%. Now,do I lose speed or torque or both through the inefficiencies? If i have a 20:1 gear reduction (motor coupled to pinion driving ring gear coupled to shaft) is my speed should be exactly 20 times less, so I should then be losing torque?

What exactly causes a motor with gearbox to be less eff. than a motor only?

 

[ecampos]

In essence you are losing Power which is equal to Torque X Speed. In the gear box, you lose efficiency because the bearings have play and grease or oil and they create friction. There is power lost due to the oil being swished around. The gear mesh is important, the better they mesh, and the better shape/profile the better their efficiency. Remember that a motor is never 100% efficient, and most motors only have high efficiency at some small range of speed/torque. It has its own losses when compared to the power supply. The motor itself heats up the gear box and reduces efficiency. You should design the motor/gearbox assy so that its efficiency is something that yo can handle at the loads that you are moving. In other words oversize it.

Edson Campos
edsoncampos@earthlink.net

 

[smitty1322]

ecampos,
Thank you for your reply. I should have added the fact that I am trying to size my motor by calculating backwards through the gearbox.
I know what torque I need at my output to move the load at the customers specifications. I have an electric motor that will do the job. What I am wondering is if I have to redesign my motor to take into account the inefficiency of the gearbox? I know the gearbox is not 100% efficient, so i have to redesign my motor, but do I redesign for speed or torque?

 

[ecampos]

Either improve your gearbox efficiency or increase the power of your motor to overcome that. In a hospital bed, you probably are not out for high speed operation, so calc for torque output. Remember that Torque is the cause and speed is the effect. Decide on what speed you want to drive and then tailor the motor to output a adequate torque in a good efficiency range.


Edson Campos
edsoncampos@earthlink.net

 

[BrianDengel]

Efficiency is a measure of the percentage of power that travels through a mechanism from its power source to the output. It is power is reduced by shaft misalignments, by heat generation in the gear mesh, lubricant losses in the bearings and any other friction losses. It is also effected by the inertia of a system. An ideal system which is 100% efficient would have zero friction and zero interia.

 

[smitty1322]

Everyone,

Thank you for your responses, that was what I was looking for. I have heard a rumor that a good estimate for efficiency loss is 10% per gear reduction, does that sound about right? I also saw in a university study that transaxle motors at free speed are about 70% eff. Under load that dropped as low as 45%. I just need to know some general information like that. If anyone has any I would love to hear. Thank you for your help.

Smitty