PTO drive for a Miller Bobcat welder?

[493mike]

I was given a portable welder with engine trouble (rod through crankcase) and think I can adapt it to PTO drive to use with my B2920. How fast must I spin the engine to get enough HP/torque to spin the generator at 3600 rpm? The old engine was a 16 HP Onan. I plan on increasing the speed with V-belts/ pulleys.
Mike

 

[mcfarmall]

There should be a mark on the tachometer which reads 540 PTO or something similar. Run the engine there and do the math mechanically to increase the speed up to 3600 rpm. Gearbox, different size pulleys are two common methods to change speeds.

 

[JasonW]

Most are 3600 rpm or 60hz. Be aware if you get it set up with belts at the correct RPM/HZ and the genset is under load it may slip bringing the HZ down with it. Could damage the genset.
Replacing the engine isn’t an option?

 

[GreensvilleJay]

The big issue is HOW are you going to control generator speed ? Most (all ? ) welders have current sensors to control engine RPM ?
As for PTO to drive it, you'll need a x6 increase( say 3" PTO pulley, 18" on the generator. I fgeeny doesn't have one, add a 'frequency' meter , you'll want to get RPM set to read 60-62Hz.
Cheapest way of course, either rebuild or replace the Onan.

 

[Donystoy]

The big issue is HOW are you going to control generator speed ? Most (all ? ) welders have current sensors to control engine RPM ?
As for PTO to drive it, you'll need a x6 increase( say 3" PTO pulley, 18" on the generator. I fgeeny doesn't have one, add a 'frequency' meter , you'll want to get RPM set to read 60-62Hz.
Cheapest way of course, either rebuild or replace the Onan.

I think you have the pulleys reversed. You would want the 18" on the PTO and the 3" one on the generator. I certainly would not go this route and agree that the Onan should be rebuilt or another engine sourced.

 

[JasonW]

Just looked at a pulley calculator. 540PTO 18” with a 3” motor side is 3240RPM

Pulley Size & RPM Ratio Calculator | Compressor Source

Use Compressor Source’s Pulley & RPM Ratio Calculator to determine shaft speed or pulley size for belt-driven mechanical systems, ideal for air compressors.

compressor-source.com

 

[InTheWoods]

I was given a portable welder with engine trouble (rod through crankcase) and think I can adapt it to PTO drive to use with my B2920. How fast must I spin the engine to get enough HP/torque to spin the generator at 3600 rpm? The old engine was a 16 HP Onan. I plan on increasing the speed with V-belts/ pulleys.
Mike

If you ran the engine near the 'rated' 2800 RPM speed and used the 960 RPM PTO gearing, you'd have 21HP available, which is more than enough. You could probably run the engine a touch slower and still have enough HP, but you'd need a higher pulley ratio, which is undesirable. Using the 540 RPM PTO gearing would be nuts for this application.

3600/960 is still almost 4:1 - which is do-able, but transferring 16HP with V-belts takes some careful attention to details.

 

[North Idaho Wolfman]

You can not make it work of the PTO of a B2920!
At 540RPM you have 21HP available.
If you reduce that by 6.6 times you end up with 3.18 HP available.
That doesn't even take into account the amount of loss you get with belts, pulleys and bearings.

Get this.

https://www.harborfreight.com/22-hp-670cc-v-twin-horizontal-shaft-gas-engine-epa-61614.html?hftm_sc=3145&utm_source=google&utm_source=google&utm_medium=cpc&utm_campaign=22877905593&campaignid=22877905593&utm_content=184478323820&adsetid=184478323820&product=61614&gad_source=1&gad_campaignid=22877905593&gbraid=0AAAAADAHb4duaoV8l2Rpn1i3t9_1bz90X&gclid=Cj0KCQjwjL3HBhCgARIsAPUg7a4GfnIDz6enpxgzF97CeOQEm4QJRzaQOS96rtJwvpU3wS3FF9r_jpIaAtioEALw_wcB

 

[PoTreeBoy]

While I don't agree with Wolfman's hp analysis, I do agree with his conclusion.

You need a 1:6+- ratio. You're not going to drive 16 horsepower through a single v-belt and 3 or 4" sheave at 3600 rpm. Even then you'd need double-reduction to keep the drive pulley a decent size. And, a v-belt drive like that is real inefficient.

Your best bet would be to find a drive off a burnt-out PTO generator. Generator drives typically have tapered shafts and the generator relies on the engine (or drive) to support the drive end. I'm not sure about your welder, but I suspect it's the same. If you buy an engine be prepared for that.

 

[InTheWoods]

You can not make it work of the PTO of a B2920!
At 540RPM you have 21HP available.
If you reduce that by 6.6 times you end up with 3.18 HP available.

More certain than correct again...

 

[torch]

HP = speed x torque. You can't change one without affecting at least one other. Six times more speed means six times less torque.

 

[InTheWoods]

^right. And HP remains constant (exclusive of frictional loss).

 

[GreensvilleJay]

I think you have the pulleys reversed. You would want the 18" on the PTO and the 3" one on the generator. I certainly would not go this route and agree that the Onan should be rebuilt or another engine sourced.

oopsy... getting old sux.... ,yes 18 on PTO, 3 on genny to spin the genny fast.

 

[InTheWoods]

Again, use the 960 PTO setting and all you'd need is a 11.25 on the PTO (with your 3" on the generator).

 

[North Idaho Wolfman]

HP = speed x torque. You can't change one without affecting at least one other. Six times more speed means six times less torque.

Yes I said HP but i really meant Torque, torque loss is the real issue with massive multiplication of speed.

 

[torch]

Yes I said HP but i really meant Torque, torque loss is the real issue with massive multiplication of speed.

Ultimately you were not wrong in that one cannot magically increase the entropy of the system by changing pulley (or gear) ratios. One can only convert, trading one characteristic for another. Where one characteristic goes up, another goes down. So let's look at the specifics of this case:

An engine spinning at 3600 rpm requires 23 ft-lbs of torque to make 16 HP. (Theoretically.)

A PTO spinning at 540 rpm provides 204 ft-lbs of torque to make 21 HP. (Again, in theory.)

So if we increase the speed by a factor of 6.6, we decrease the available torque by a factor of 6.6. IE 204 ft-lbs @540 becomes 30.9 ft-lbs @3600.

And 30.9 ft-lbs spinning at 3600 rpm is <insert drum roll here>: 21 HP. Entropy is preserved.

Of course, in the real world there will be lots of energy "lost" (ie: radiated as heat) due to friction of the belts on the pulleys and shafts in the bearings, etc. But there might be enough surplus to end up with the required 16hp input to the generator.

I think the bigger problem with this plan is the lack of regulation. As others noted above the genset as a unit regulates engine speed and power based on demand. With no real feedback to the tractor engine I suspect the power produced will be quite "dirty" -- voltage and frequency will fluctuate with load. Could be used in some applications, but whatever is being powered better be very tolerant or very constant.

 

[lynnmor]

I think the bigger problem with this plan is the lack of regulation. As others noted above the genset as a unit regulates engine speed and power based on demand. With no real feedback to the tractor engine I suspect the power produced will be quite "dirty" -- voltage and frequency will fluctuate with load. Could be used in some applications, but whatever is being powered better be very tolerant or very constant.

I believe the speed setting on the engine will regulate the RPM to what it was set. Under heavier loads the engine will be governed by increasing fuel. Correct me if I am wrong.

 

[torch]

I believe the speed setting on the engine will regulate the RPM to what it was set. Under heavier loads the engine will be governed by increasing fuel. Correct me if I am wrong.

If the tractor has a governor that can be set that way, sure. That's how the genset works.

My tractor doesn't have such a governor but others might.

 

[McMXi]

I think the bigger problem with this plan is the lack of regulation. As others noted above the genset as a unit regulates engine speed and power based on demand. With no real feedback to the tractor engine I suspect the power produced will be quite "dirty" -- voltage and frequency will fluctuate with load. Could be used in some applications, but whatever is being powered better be very tolerant or very constant.

This sums it up. I would say either repair/replace the engine or convert the welder to run off 220V (if that's an option). The lack of regulation of engine speed of the tractor is going to be problematic. CC or CV are required depending on welding process so I don't see this working out well at all.

I almost bought a Miller Bobcat many years ago when the top of the line models were in the $6k range. Now they're $25k or so, and depending on the model of the Bobcat that the OP was given, a replacement engine might be well worth the cost.

 

[PoTreeBoy]

I believe the speed setting on the engine will regulate the RPM to what it was set. Under heavier loads the engine will be governed by increasing fuel. Correct me if I am wrong.

Our tractors do have governors, but they don't have the same characteristics as power generation governors. A generator has very little speed droop from no load to full load so that its frequency change is low. Our tractors have more droop, so wouldn't have good frequency control over the full load range. However, if the generator's load was a small fraction of a tractor's rating, it could give acceptable results.