# Measuring mechanical torque of a generator while running

#### strantor

Joined Oct 3, 2010
6,685
I've posted about my generator project a couple of year ago; it's sat idle from then until last week. It's going to be a PTO generator, powered by the rotating shaft coming out of the back of my tractor. This is the typical arrangement (not my picture):

This is where I'm at so far:

I want my generator to serve double duty as a PTO dynomometer.

Please don't suggest I measure electrical power output and calculate mechanical load. That is not what I want to do.

I want to measure physical torque and physical RPM of the PTO shaft.

So here's the idea I'm playing with (side plates of belt drive assembly shown in blue glass for clarity, actually steel):

Instead of a gearbox, I'll have a belt drive, and instead of being rigidly bolted, it won't be fastened to anything. It will be "free"/"floating," hanging from bearings off of the generator input shaft, such that if I were to hang a pipe wrench off the PTO shaft and put some weight on it, (assuming the generator rotor were locked or providing some appreciable resistance) the whole belt drive assembly would rock to the side:

So if I were to put a load cell (red cylinder) exactly level with the PTO shaft:

since the center distance between pulleys is exactly 1ft, that load cell should read out directly in ft*lbs! Right? of course? ...

The more I think about it, the less convinced I am, but I'm not sure why. My intuition-only physics is failing me; I need some educated physics feedback.

My confidence starts to falter when I ponder whether I would be measuring torque before losses or after. There will be some losses in the belt drive. Would this setup yield the torque output from the tractor's PTO, or the torque experienced by the generator shaft? and spiderweb cracks in my understanding branch out from there.

Depending on answer to that, how do the losses manifest? Is there a counteracting force from the generator shaft?

How does this tite-reach ratchet extender work? and why? Does its existence shoot my idea out of the water?

Would this even work at all, forgetting about losses?

#### Sensacell

Joined Jun 19, 2012
3,257
Hmmm. it looks like your mechanical arrangement requires the INPUT shaft to float, which is where the idea falls apart.

How can the assembly rotate against the load cell while the input shaft is coupled to the tractor, which it must be?

The belt drive ratio is meant to increase the generator RPM? yes?

Joined Feb 20, 2016
4,314
I think you would do better to mount the generator on a rotatable bearing, like that of a car real wheel assembly. That would be strong enough. This keeps the center point constant.
Then, have mechanical limits on the rotation, with springs to work against.
When the generator is powered up, increasing the load will cause the generator to rotate against the springs. Measure the rotation with a load cell or even a shaft encoder to determine the torque.
The spring could be an attached cable wrapped around the generator then passing through a coil spring to allow a fair length of travel.

That is just one way to do it.

#### drc_567

Joined Dec 29, 2008
1,156
Dynomometer
...
... The usual method of measuring output power is to contrive a way to apply a braking force to the rotating output shaft. The force magnitude and distance from the tangent contact point, constitute a torque value, which together with the observed shaft RPM, provide sufficient information to calculate the mechanical power. (see link above) The braking mechanism might consist of a wooden beam, resting on the output shaft, and supporting a known weight at a given distance from the contact point.
Prony Brake

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#### strantor

Joined Oct 3, 2010
6,685
The belt drive ratio is meant to increase the generator RPM? yes?
Yes.
Hmmm. it looks like your mechanical arrangement requires the INPUT shaft to float
Can you expound/explain why? I thought the same at first but then convinced myself that the floating belt drive assembly is a 12" lever, and the generator input shaft can be thought of as its fulcrum. It can be easier realized if you mentally extend the belt drive assembly another 12" upward.

Traditionally we would describe it more in the way that (I think) you think of it, with the PTO shaft being the fulcrum and the generator input shaft being the measurement point, but in (my) theory, torque is a relative measurement like voltage and any measurement between two points is valid as long as the points are known. force could be measured anywhere along the lever and whether the "fulcrum" exists at the PTO shaft or the generator shaft is only a matter of convenience/semantics.

To further the voltage analogy, It is easiest to think of the fixed point as the fulcrum (ground) and floating point as the measurement point (V1). But we can reverse the test leads and get the same measurement (corrected for polarity). Or we can move the ground point from the generator shaft to the PTO shaft. Or both. Either/any way, the potential difference still exists and can be measured.

I'm not trying to educate you; I'm trying to explain my (probably flawed) understanding of the situation in enough detail that you can pinpoint where is the flaw in it. So please, feel free to hack away at my words until I get smarter.

#### strantor

Joined Oct 3, 2010
6,685
I think you would do better to mount the generator on a rotatable bearing, like that of a car real wheel assembly. That would be strong enough. This keeps the center point constant.
Then, have mechanical limits on the rotation, with springs to work against.
When the generator is powered up, increasing the load will cause the generator to rotate against the springs. Measure the rotation with a load cell or even a shaft encoder to determine the torque.
The spring could be an attached cable wrapped around the generator then passing through a coil spring to allow a fair length of travel.

That is just one way to do it.
That's the way that I wanted to do it originally but the design got too complicated. I don't know if the pictures convey the scale well to someone who hasn't seen the generator in person, but it weighs nearly 2,000lb/900kg. It's over 2ft in diameter and 4ft long. In order to hang it from a "rotisserie" whose axis passes through the center of the generator shaft, well it would be a feat of engineering.

Alternatively I had considered setting it on casters, concept similar to this:

But I have used roller spoolers like this in real life and can say that the force required to get the spool to move is great. I think the friction/resistance would skew my measurements significantly. Maybe not so bad since the generator is smooth steel as opposed to uneven wood, but it is a big investment in time to find out if something that I already think won't work, actually won't work.

Another option I considered along the same lines is to weld a hinge along the bottom length of the generator, such that it's free to flop side/side, and pin it at the top with a load cell. This would require the generator to be parked on a perfectly flat surface to work properly, and it would require me to spend an uncomfortably long time underneath the precariously suspended man-crushing generator. The only way I can lift this is with my tractor, and the lifting setup is not something that inspires any confidence at all. The hinge could be along the top, but then we're back to the rotisserie structure.

My fallback idea is to park the generator's wheels on two scales, tare the scales before operation, and measure how much weight is taken off one wheel and added to the other. I would really rather have the measurement be part of the assembly though, and everywhere I can think to put load cells on the unit to achieve this concept, would Subject them to shock loading every time I move it. Also the math required to turn that weight shift at ground level into a torque value at PTO shaft level might not be very straightforward.

#### strantor

Joined Oct 3, 2010
6,685
Dynomometer
...
... The usual method of measuring output power is to contrive a way to apply a braking force to the rotating output shaft. The force magnitude and distance from the tangent contact point, constitute a torque value, which together with the observed shaft RPM, provide sufficient information to calculate the mechanical power. (see link above) The braking mechanism might consist of a wooden beam, resting on the output shaft, and supporting a known weight at a given distance from the contact point.
Prony Brake
Your post says "Last edited by a moderator: Today at 4:47 AM." Not sure what was edited out, but it must have been the part that made it relevant. Obviously I know what a Dynomometer is or I wouldn't be building one. Are you suggesting I build a pony brake separate from the generator? I don't want to do that.

#### strantor

Joined Oct 3, 2010
6,685
I've been playing with a mental model of the tite-reach ratchet extender (wish I had a real one, might get one) and I think if I replaced my belt drive with a massive tite-reach ratchet extender, it would not swing to the side as I showed in the OP. At least an ideal model wouldn't. The real deal probably would, a little, but only because of the backlash of the gearing inside. IOW the losses.

Swapping my belt drive back in for the massive tite-reach, I think the same is probably true. The force experienced by my load cell would only represent that left-over force which was not transmitted to the generator shaft due to belt slip or other mechanical inefficiencies.

Does that sound about right? If so, then does the ratio effect it at all? The tite-reach ratchet extender is a 1:1 gear train. My belt drive is 1:3.33. I think that still probably doesn't matter. It would be like coupling a tit-reach to a force multiplier. The outcome would be the same (I think)

#### drc_567

Joined Dec 29, 2008
1,156
Your post says "Last edited by a moderator: Today at 4:47 AM." Not sure what was edited out, but it must have been the part that made it relevant. Obviously I know what a Dynomometer is or I wouldn't be building one. Are you suggesting I build a pony brake separate from the generator? I don't want to do that.
... The edit must have been minor ... most of it is there, as I can recall.

... In order to perform the power measurement, some form of lever type brake would be sufficient. That is, a hinged, rigid, substantial, length of lumber should be able to rest on the output shaft of the tractor. At the free end of a hinged lumber beam, a reasonable weight can be affixed. This is the force, F. The length between the tangent beam contact point with the rotating tractor PTO shaft and the weight is the distance, say 'd'. Then F*d=T, the required torque. Some way to measure the output shaft RPM is required.
... An elaborate brake assembly, as described in the reference links above should not be necessary.
If the general idea is not getting across, a sketch or drawing might be necessary. This method involves measuring the actual mechanical variables present at the output shaft. As applied to your objective, this method would be used to measure the mechanical power of the tractor PTO shaft that drives the generator. This measurement would be the upper limit of the power capability if the generator. Subsequent measurements of the generator Voltage and Current would give a value for electrical power. The ratio of actual electrical power to mechanical power would be a number for overall efficiency.

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#### Sensacell

Joined Jun 19, 2012
3,257
For the sake of clarity, I am naming the two shafts: (please correct me if I got this wrong)

Low-speed input shaft: large belt pulley, this connects rigidly to the tractor PTO shaft.
High-speed generator shaft: small belt pulley, drives generator input shaft.

To have a hope of accurately measuring the torque applied to the "Low-speed input shaft" - all the reaction forces must be seen by the load cell.
If the whole belt transmission is free to rotate about the "High-speed generator shaft" this implies that the center axis of the "Low-speed input shaft" must be free to move.
(even if it's a small amount, it must be free to move, and the load cell must provide the only resistance to this motion)

The bearings on the tractor PTO would absorb the force and the load cell would not read correctly.

Does this sound correct?

It's an interesting problem.

#### strantor

Joined Oct 3, 2010
6,685
For the sake of clarity, I am naming the two shafts:
Good call, and I should start labeling them in my pictures. The names you chose are good descriptors but they're going to take up a lot of real estate in my pictures so I'm going with letters.

(please correct me if I got this wrong)
Low-speed input shaft: large belt pulley, this connects rigidly to the tractor PTO shaft.
I'll be labeling this "Shaft B" henceforth.
Almost Correct. It doesn't connect rigidly. It connects via a double-cardan driveshaft like this:

High-speed generator shaft: small belt pulley, drives generator input shaft.
Correct. I'll be labeling this "Shaft A" henceforth.

To have a hope of accurately measuring the torque applied to the "Low-speed input shaft" - all the reaction forces must be seen by the load cell.
If the whole belt transmission is free to rotate about the "High-speed generator shaft" this implies that the center axis of the "Low-speed input shaft" must be free to move.
(even if it's a small amount, it must be free to move, and the load cell must provide the only resistance to this motion)

The bearings on the tractor PTO would absorb the force and the load cell would not read correctly.

Does this sound correct?

It's an interesting problem.
It sounds correct assuming the rigid connection to the "Low-speed input shaft" (Shaft B), however since it's it's a floppy driveshaft I think there will be plenty of "play" for the purpose of measurement.

#### shortbus

Joined Sep 30, 2009
9,938
How does this tite-reach ratchet extender work? and why?
I can answer these questions. The tite-reach is an adaption of another product called a Steer Clear. They are made for street/hot rods to get the steering column out of the way for engine swaps. It/they use two sprockets and a chain to move the points of rotation. Using a chain works to keep the direction of rotation consistent. If gears were used it would mean you would need to have an odd number in the gear train, or rotation would be the opposite direction. There was at one time a look at the internals of the Steer Clear but I can't find it now. http://www.wizardsteerclear.com/

Now why are you wanting to do this measurement? Is it to test other tractor PTO's? Doesn't make sense to just do it for your working set up, or at least not to me. But am glad to see you used my idea of the belt drive, from the other thread on this. Using your set up as a dyno doesn't make sense to me because unless you have a way of calculating your drive losses any numbers will just be an approximation, and the PTO numbers are or were available from tractor makers.

#### strantor

Joined Oct 3, 2010
6,685
In order to perform the power measurement, some form of lever type brake would be sufficient. That is, a hinged, rigid, substantial, length of lumber should be able to rest on the output shaft of the tractor. At the free end of a hinged lumber beam, a reasonable weight can be affixed. This is the force, F. The length between the tangent beam contact point with the rotating tractor PTO shaft and the weight is the distance
Ok, I agree... You've just described a pony brake. In my case, the generator is the brake. I don't see the connection between the generator and the pony brake.

As applied to your objective, this method would be used to measure the mechanical power of the tractor PTO shaft that drives the generator. This measurement would be the upper limit of the power capability if the generator. Subsequent measurements of the generator Voltage and Current would give a value for electrical power. The ratio of actual electrical power to mechanical power would be a number for overall efficiency.
So are you suggesting I measure the mechanical power output of the tractor with a pony brake and then measure the electrical output of the generator? If so, that's not my objective. I want the generator to serve as a generator and a dynomometer, at the same time.

#### strantor

Joined Oct 3, 2010
6,685
I can answer these questions. The tite-reach is an adaption of another product called a Steer Clear. They are made for street/hot rods to get the steering column out of the way for engine swaps. It/they use two sprockets and a chain to move the points of rotation. Using a chain works to keep the direction of rotation consistent. If gears were used it would mean you would need to have an odd number in the gear train, or rotation would be the opposite direction. There was at one time a look at the internals of the Steer Clear but I can't find it now. http://www.wizardsteerclear.com/

Now why are you wanting to do this measurement? Is it to test other tractor PTO's? Doesn't make sense to just do it for your working set up, or at least not to me. But am glad to see you used my idea of the belt drive, from the other thread on this. Using your set up as a dyno doesn't make sense to me because unless you have a way of calculating your drive losses any numbers will just be an approximation, and the PTO numbers are or were available from tractor makers.
Thanks for the belt suggestion. I was opposed to it initially mostly because of the cost of the large multi-groove pulley, but I ended up finding one on eBay recently for very cheap.

I'm wanting a PTO dyno because I intend on hacking my tractor and I want a way to quantify the results. I want to confirm/refute the PTO HP value provided by the manufacturer, and then I want to download firmware from other tractors into it and see if I get any gains. My tractor is 40HP but to my knowledge is mechanically identical to the 45HP, 50HP, and 55HP versions of it sold at higher prices. The only difference that I'm aware of is the programming in the ECU.

Also because I'm curious how efficient it is to spin up this gigantic generator just to provide the puny 130A needed to run my all-electric central HVAC plus well pump, septic pump, water heater, and lights to take an illuminated hot shower in an ice storm.

#### strantor

Joined Oct 3, 2010
6,685
I've been playing with a mental model of the tite-reach ratchet extender (wish I had a real one, might get one) and I think if I replaced my belt drive with a massive tite-reach ratchet extender, it would not swing to the side as I showed in the OP. At least an ideal model wouldn't. The real deal probably would, a little, but only because of the backlash of the gearing inside. IOW the losses.

Swapping my belt drive back in for the massive tite-reach, I think the same is probably true. The force experienced by my load cell would only represent that left-over force which was not transmitted to the generator shaft due to belt slip or other mechanical inefficiencies.

Does that sound about right? If so, then does the ratio effect it at all? The tite-reach ratchet extender is a 1:1 gear train. My belt drive is 1:3.33. I think that still probably doesn't matter. It would be like coupling a tit-reach to a force multiplier. The outcome would be the same (I think)
I've been thinking more about this and I think I disagree with myself. I drew up something else to illustrate my point, and in the process I think I actually refuted it. I abandoned the abstract and unfamiliar (probably to most, except @shortbus) example of the tite-reach ratchet extension, and went to something more basic and intuitive: a mechanical linkage. Here's the Graphic:

(assume the rotor (shaft A) is locked)

If the linkage were ideal (absolutely zero play in the linkage pins) I believe the force in the direction of the blue arrow would be zero. However, if there were appreciable slop in those pins, there would be a quantity of force. In my quoted section above the concept that I posited dictates that this force would be minimal, only equal to (or proportional to) the amount of loss/slop in the linkage pins. However, after looking at it, I do not believe that is the case. I believe that the full applied torque would be transmitted in the direction of the blue arrow (until that slop is taken up). I now think that it is precisely because of that slop, that the idea would work. And since the the rotor will be rotating, and since the belt drive has some slippage/stretch, the slop would be perpetually replenishing, never taken up.

I think this will work. Any disagreement on that?

Furthermore, I believe that I will be measuring actual torque, before losses, of the tractor PTO shaft (shaft B).

Any disagreement on that?

#### shortbus

Joined Sep 30, 2009
9,938
My tractor is 40HP but to my knowledge is mechanically identical to the 45HP, 50HP, and 55HP versions of it sold at higher prices. The only difference that I'm aware of is the programming in the ECU.
That may be true today, but in the old days it was usually due to higher compression. Either in the cylinder head or pistons, in the pistons it was the distance form the top of the piston to the wrist pin location.

To measure what you want to know from what you have, wouldn't it be better to feed the output of the alternator/generator, if it's AC output technically it is an alternator, into some type of resistance? That is how most modern dynos do it, though they use a eddy current generator to a water cooled resistor. Or a water pump and restriction of the output of the pump.

You do understand though that dynos only measure torque not HP. The HP is figured from the torque by a math formula.

#### shortbus

Joined Sep 30, 2009
9,938
@strantor, you posted your drawing while I was writing. Do you have a flat belt pulley for the tractor? If you do or can find one you are half way to a prony brake. A couple of wood wood blocks with half the diameter cut into each one and an arm with a way to tighten the blocks to put pressure on them and a scale and you have a prony brake. https://en.wikipedia.org/wiki/Prony_brake

Or another idea would be to adapt an old brake disk to the PTO shaft, then use a caliper and a master cylinder to power the caliper. The master cylinder could also be a pump from a portapower. The caliper would then be attached to an arm going to the scale.

#### strantor

Joined Oct 3, 2010
6,685
That may be true today, but in the old days it was usually due to higher compression. Either in the cylinder head or pistons, in the pistons it was the distance form the top of the piston to the wrist pin location.
There is only one parts manual for the whole line of tractors, and it doesn't show any separate P/N for heads, cranks, or pistons, fuel pumps, injectors, or any other mechanical difference (that I've found) between the different models. It does show different P/N for the ECU though. I have found that this "software handicap" strategy is actually pretty common. I guess it's more cost effective for the manufacturer to have only one production line, produce only one widget, and sell it at multiple tiers of [price : quantity], than it is to actually produce several different widgets with variable levels of functionality, on several production lines. For example my FLIR E4 thermal camera (80x60pixels, $1,000) is physically identical to the E5 (160x120px,$1,400), E6 (240x180px, $2,000), and E8 (320x240px,$3,000). What happened to the E7? They decide it was just "too much?" I put E8 firmware into my E4 and now it magically has 320x240 pixel resolution.

To measure what you want to know from what you have, wouldn't it be better to feed the output of the alternator/generator, if it's AC output technically it is an alternator, into some type of resistance? That is how most modern dynos do it, though they use a eddy current generator to a water cooled resistor. Or a water pump and restriction of the output of the pump.
Most modern tractor dynos, to my knowledge, employ a hydraulic pump and they just flow fluid over an increasing value of relief pressure to measure maximum torque.

Most modern automotive dynos, to my knowledge, don't have any kind of adjustable resistance. They're just a huge flywheel and calculate the power based on how long it takes to accelerate it. Moment of inertial.

I've never seen or heard of a dyno with a generator coupled to a load bank, but it makes sense, and that's effectively what I'll be doing here, but I don't want to calculate PTO HP based on electrical load. there are too many variables, too many things that could change from day to day, test to test. Accuracy of electrical measurement device, angle of the driveshaft, ambient conditions, differences in the efficiency of the generator at different torques, speeds, and temperatures, differences in the efficiency of the belt drive depending on wrap angle, tension, and wear, etc. It could probably be "reasonably" accurate if it were proven and calibrated against a known prime mover, but I don't have one.

The only way I can know that my numbers are good, is to put a load cell tangent to the axis of PTO and measure the load. Like a pony brake.

You do understand though that dynos only measure torque not HP. The HP is figured from the torque by a math formula.
Yes I'm aware. HP = (RPM * FT*LBS)/5252. I'll be measuring RPM as well, and doing the math.

@strantor, you posted your drawing while I was writing. Do you have a flat belt pulley for the tractor? If you do or can find one you are half way to a prony brake. A couple of wood wood blocks with half the diameter cut into each one and an arm with a way to tighten the blocks to put pressure on them and a scale and you have a prony brake. https://en.wikipedia.org/wiki/Prony_brake

Or another idea would be to adapt an old brake disk to the PTO shaft, then use a caliper and a master cylinder to power the caliper. The master cylinder could also be a pump from a portapower. The caliper would then be attached to an arm going to the scale.
Yes there are all sorts of ways to make a tractor dyno. Pony brake, brake caliper, hydraulic pump, giant flywheel, water pump, air compressor, truck clutch, airboat fan, lower a flail mower into a pond, etc. the list endless. The way I want to do it is with this generator. I don't want to have two separate attachments. Tractor attachments take up a lot of room. I will though, if I decide that the generator-as-a-dyno idea won't work, or won't work well. I'll make a pony brake or something. But I want to give this generator it's due diligence before I resort to that.

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#### shortbus

Joined Sep 30, 2009
9,938
I've never seen or heard of a dyno with a generator coupled to a load bank,
It's called an AC dyno. But I see now that Superflow has gone to air cooled heat sinks now. https://superflow.com/product/ac-engine-dynamometers/

Most modern automotive dynos, to my knowledge, don't have any kind of adjustable resistance. They're just a huge flywheel and calculate the power based on how long it takes to accelerate it.
And I've never heard of that type. All of the ones I'm familiar with are of 3 type, hydraulic(usually water not oil), eddy current, or AC.

I still don't understand what your trying to do though. You say you want to find the increases of changes hot rodding your motor, but don't trust the electrical output of the generator to do it. But you want to use the generator as a load/dyno? Usually the PTO will start spitting out gear teeth before any appreciable load on the motor is reached. But then again I don't know about modern tractors, just the old school type. I don't think the PTO will tell you much myself.

Either a chassis or strictly engine dyno will though. Pulling and testing the motor since yo uhave a desil would be a chore. Though towing the tractor and going to a chassis dyno, there are quite a few in TX, some for semi's that may work. https://www.google.com/search?clien...&ved=0ahUKEwjl5LLiorjvAhVCBs0KHXnkAtIQ4dUDCA0

#### strantor

Joined Oct 3, 2010
6,685
It's called an AC dyno. But I see now that Superflow has gone to air cooled heat sinks now. https://superflow.com/product/ac-engine-dynamometers

And I've never heard of that type. All of the ones I'm familiar with are of 3 type, hydraulic(usually water not oil), eddy current, or AC.
Sorry, didn't realize you were talking about engine dynos. I was talking about drive-on chassis dynos. I don't know anything about engine dynos.

I still don't understand what your trying to do though. You say you want to find the increases of changes hot rodding your motor, but don't trust the electrical output of the generator to do it. But you want to use the generator as a load/dyno?
Ok let's say you're using a dyno which has an airboat propeller as its load. Do you measure windspeed off the prop and convert that to a rough HP value with some empirical equation that doesn't factor in ambient wind, or do you directly measure rpm and torque, the two values that together form the very definition of horsepower?

Or let's say you're using a dyno which has a brake disk as its load. Do you measure heat rise off the disk and pads and convert that to a rough HP value with some empirical equation that doesn't factor in friction coefficient, or do you directly measure rpm and torque, the two values that together form the very definition of horsepower?

Or let's say you're using a dyno which has a water pump as its load. Do you measure PSI and GPM and convert that to a rough HP value with some empirical equation that doesn't factor in seal leakage, or do you directly measure rpm and torque, the two values that together form the very definition of horsepower?

You see where I'm going with this? Just because I have volts and amps doesn't mean I have the data necessary to declare xxHP.