Belt Pull Force Calculation 1

[cfordyce05]

I'm trying to confirm what values I would use to calculate belt pull for a generator driven by a timing belt with a 2-stroke engine. The engine has a propeller on one end and the pulley for the generator on the other. All of the resources I have found for calculating belt pull use the full motor HP and torque values at a given rpm. I was told that I should only be using the torque required to overcome the magnetic and drag forces in the generator assembly to calculate the belt pull. Is this correct?

My goal with these calculations is to determine the correct bearing size in the generator for the load. The bearing life will be drastically different using the engine torque vs the generator torque, which is much less.

Thank you.

Casey

 

[MiketheEngineer]

Really not knowing a lot about this - why don't you start by looking at the load the generator needs to give 100% output??

 

[cfordyce05]

I was told that the torque required by the generator at full load is 28 lb-in.

 

[rb1957]

the belt is taking power from the propeller shaft to drive the generator. so i think generator torque is the right # to use.

if the belt was driving the motor, or was the onl thing driven by the motor, then you'd use motor torque. but most of the motor torque is driving the propeller, yes? like the belt drives in your car, they're only transfering the power to drive the accessory, yes?

 

[cfordyce05]

rb1957,

You are correct. Most of the torque would be driving the propeller. So for an accessory like a generator or alternator, the only power transmitted through the belt is the power required to turn the accessory?

 

[dvd]

At 28 lb-in the power transmitted with the belt is fairly low so it doesn't sound like a heavy-duty belt drive. Is the belt tension pre-set at the factory and never field-adjusted? How will the belt tension be determined? I would probably design bearings to account for a higher belt tension than required for power transmission to account for human factors.

 

[cfordyce05]

We set the belt tension, as we are the ones building the assembly. The belt tension is determined by deflecting the belt a set distance and measuring the force required to do so.

 

[satchmo]

To get in the ballpark divide the torque by half the pitch diameter. That will give you the effective belt pull.

 

[Tmoose]

Well, I'd think about what might be required for accelerating things up to speed, and add a healthy margin for higher than intended belt tension (it's pretty easy to create 100s of lbs of belt tension with adjusting screw tensioners, and thermal expansion of components can add up too ), and try to keep the load under the endurance limit of the bearing (conservative value published by SKF, Co/8 per FAG).

 

[M98Ranger]

I don't know about belts for a generator, but when you design for materials handling the belts are rated for up to 150% of the nominal tension for short periods of time (i.e. start-up). So you might find out whether standard practice dictates that they can take a specific start-up torque or not. I would guess that they do allow for start-up torque.

 

[electricpete]

For vee belts, a typical "belt factor" is 1.5 to 2.0.

This is not a safety-ractor that we apply, but a factor by which we multiply the force computed from power, speed and radius (F=P/2/pi/f/R in SI units) to estimate the actual force on the bearing while the machine is under load. Any safety factor we envision would be above and beyond this.

You can also calculate a static (Non-running) force from the belt tensioning procedure. If you apply a force F to move the belt a distance D perpendicular to the belt, then the static tension is
T = F * (CtoC/D)/4 where CtoC is sheave center to center distance. (simple geometry problem). Multiply this by 2 to get the force on the bearings in static condition in the static state immediately after tensioning.

=====================================
(2B)+(2B)' ?

 

[Cockroach]

Yeah, I tend to agree with TMoose statement, think about the dynamics also. 28 in lbf is the sustaining torque required to turn the shaft and propeller sub assembly, I presume. That would also include the friction in the bearings, etc.

But what about gunning the engine and accelerating the mass this propeller is working against? Certainly that would increase the load on the shaft and propeller sub assembly. So you would need to determine the belt size and performance against the maximum output of the engine, no?

So then the belt is sized against the engine, the propeller system is checked against such input torque performance.

That would be one way of looking at the problem. The system can only handle the maximum engine output, then add your safety factor to acknowledge worst case.

Regards,
Cockroach