Homemade Chainsaw Dynamometer

[bcmktg]

I've had the thought of building an inexpensive chainsaw "dyno" rolling around in my beady little brain for awhile now. Here's what I'm thinking....

Find a gas-powered generator with a bad engine. > Remove the engine and fabricate a means of driving the generator with a chainsaw chain (cutters removed). > Wire the output of the generator to a rheostat of sufficent current rating > to vary, and measure, the current to an electric motor > hooked to some sort of load like a boat propeller in a barrel of water. You would not be able to measure actual "horsepower", per se, but together with a tachometer on the saw, you sure as heck would be able to compare different brands and models of saws at different rpm's and load conditions. You would also be able to set a baseline with an unmodified saw, modify it, and then compare the results of the modifications. I think this setup could also be used to set top end rpms safely and more accurately under a very slight load by comparing it with a brief "dry rev" rather than more extended periods of dry rev. Once again, I believe getting a baseline to work from would be the key.

Anybody got any thoughts on this? Anyone done something the same or quite similar they would like to share?

Al

 

[Trigger-Time]

First thing that comes to mind is how hard it would be to mount
the different brands and sizes of saws. Also a saw dose not have
enough torque to start any kind of load. For instance with out
the chain moving, put it on the log put a small amount of pressure
and go WOT and see what it dose.

It would be neat though to have one!

 

[PES+]

First thing that comes to mind is how hard it would be to mount
the different brands and sizes of saws. Also a saw dose not have
enough torque to start any kind of load. For instance with out
the chain moving, put it on the log put a small amount of pressure
and go WOT and see what it dose.

It would be neat though to have one!

Not exactly a good analogy. We did make some electric dynos for karts that would work for saws We did not use 110 generator heads due to the lagging of the rotating mass of the armature we used small car alternators but they did not appreciate the loads we would switch in under certain tests we had not gone far enough in that direction to think of varying the loading with the field voltage and went hydraulic mimicking the outrageously expensive water brakes.....errrrr sort of

 

[bcmktg]

First thing that comes to mind is how hard it would be to mount
the different brands and sizes of saws. Also a saw dose not have
enough torque to start any kind of load. For instance with out
the chain moving, put it on the log put a small amount of pressure
and go WOT and see what it dose.

It would be neat though to have one!

I believe that mounting the saw would be the easiest part, actually. I might be overly optimistic, but I think you could tighten the saw motor against the chain drive the same way you adjust a motorcycle chain. I figure you could place the chainsaw motor on a sliding plate with a vertical plate, or post, in front of the crankcase. After aligning the chain, you would secure the body of the saw and the handle end to prevent any movement while testing.

As far as the starting load goes, I would attempt to start the test with a slight load while bringing up the rpm's to the desired point and then begin balancing the throttle against the load until the optimum point is reached. Sort of like we used to balance the throttle of the tractor against the load of the buzz saw until we could leave the tractor unattended, without having the throttle wide open, while feeding the saw. Like I said, maybe I am overly optimistic, but I am betting it could work.

Al

 

[PES+]

Chains are not needed and can give inaccurate readings. Direct coupling will give a truer reading of what is going on. A toothed belt with a standard pulley with adapters could be used after calculating the true losses of the belt and pulleys....and an adapter for rim drive clutches would work well to test clutches.

DANG IT bc!!!.......I am in the middle of too many projects and you have to get me back to designing this in my head.....

 

[Trigger-Time]

What works in you head and on paper dosn't always work in the real world.

 

[Haywire Haywood]

That would be the ideal tuning device would it not? You could do your tuning with the saw under a consistent load that wouldn't very like it does if you put it in a log. Sounds complicated though.

Ian

 

[bcmktg]

Chains are not needed and can give inaccurate readings. Direct coupling will give a truer reading of what is going on.....

Absolutely true. But since we are only interested in comparing to a baseline and not deriving an accurate, true horsepower reading, I would think it makes little difference in this instance.

The only reason I was suggesting using a chainsaw chain with no cutters is so that you could simply mount the power head in the fixture and hook the chain to it without changing any sprocket or pulleys, etc., on the saw. Align the chain, secure the power head, and let 'er rip! If you had a choice of 3 different sprockets mounted at the same time on the generator, you could cover .325, 3/8, and .404 by simply choosing whichever flavor of chain the saw used. One of the engineering details that may have to be worked at the git-go would be the drive reduction ratio. Most of the gas generators run at low rpm's compared to the average chainsaw, most of which turn in excess of 10,000 rpm. I would think somewhere between 4 and 6 to 1 would put it in the ballpark.

We did not use 110 generator heads due to the lagging of the rotating mass of the armature we used small car alternators but they did not appreciate the loads we would switch in under certain tests we had not gone far enough in that direction to think of varying the loading with the field voltage and went hydraulic mimicking the outrageously expensive water brakes.....errrrr sort of

Once again, I am hoping the drive reduction ratio might possibly make up for the armature rotating mass problem you mentioned, but then again, maybe not. Maybe older-style automotive DC generators linked axially.... hmmmm.

Al

 

[computeruser]

Sounds like a fun winter project in the works here! I'm intrigued. Keep us posted on any progress that you make on this.

 

[Kneejerk Bombas]

How about having a rubber wheel that pivots against the outside of the clutch drum?

 

[bcmktg]

How about having a rubber wheel that pivots against the outside of the clutch drum?

I like that idea except for some saws have things kind of close around the perimeter of clutch that would make it difficult to get a rubber wheel up against the clutch shell. I surely would like to get around using the chain to drive the generator if I could, but I haven't thought of any simpler solution yet. I am going to start making the rounds of local small engine shops and equipment rental places this week and put the word out that I am looking for a generator with a bad engine. Gotta start somewhere.

Al

 

[Erick]

Inverter???????

How about using one of the old style inverters that used a DC motor to drive an internal generator. Might handle the RPM's and be less rotating mass?????
Not sure how you would mount the saw engine in place of the DC motor but I would think a simple shaft maybe with an outboard mounted bearing and a rim sprocket set up would work.

Where theirs a will theirs a way

I like the idea keep working on it.

 

[GPH85]

I think you need to drop OL' Homey an email and see how he did his, if he ever got it up and going. He was going to use a power steering pump or something of that nature to place a load and measure power and torque by. Not sure if he went with direct coupling or not or as far as that goes if he ever got around to it. Not heard from him in a while, but I'm sure he would be of great assistance on this if you just dropped him an email or pm him.

Just a thought...

 

[olyman]

along that line--guy not 20 miles from me--invented--and patented ----a small engine dyno---think briggs and struuuttin--:biggrinbounce2: -and drives it off the engine with belts--dont know why you couldnt use his style for a chainsaw--just drive it with the chain--it aint like youll be running it for 30 minutes steady---

 

[PES+]

You can....a point of fact.....all saws now are rated in kilowatts first...the standard power factor now has become electrical loading. We built more stupid contraptions in the past to test modifications than you can shake a stick at....I just now remembered why there is a belt clutch on of all things a Roper saw in my pile...nd yes I have the adapters to make it fit dang near every shaft.......in a box......... somewhere in New England :crazy1:

 

[bcmktg]

Many good ideas

Lots of good ideas and alternatives here. I have been trying to consider other ways of accomplishing the same thing and not have tunnelvision on the generator, but I keep coming back to how easy it is to measure electrical current. I guess I should learn to focus more on the project and try not to be distracted so much by the thought of my wife in chainsaw chaps. :blush:

Al

 

[drmiller100]

water pump and vary the ouput nozzle size to control pressure.

measure pressure, volume, and calculate horsepower.

 

[Tree Farmer]

water pump and vary the ouput nozzle size to control pressure.

measure pressure, volume, and calculate horsepower.

Ironic, I have been think about this for a about week now on and off. I would use a hydraulic pump with a known displacement and needle valve and pressure gauge. GPM X PSI / 1714 = Hp . The problem would be finding a high rpm pump or a large sprocket to gear it down. It would not be as accurate as a direct mounted generator unless you could figure in the HP loss thorough the chain, sprocket and pump. It would effective for tuning or before and after testing during a mod project.

PS. ordered a 5100s today cant wait to get it!

 

[Lakeside53]

You would need to dissipate the heat in the fluid. Many options..


What about an old pressure washer pump and just let it spray water into a drain?

 

[Stihl 041S]

How to test

We test pumps all the time, just sort of reverse. It would be hard to test 43000GPM at 200 foot head. Or to test a 4500 hp motor pump any direct way but as you say, constriction with pressure and oriface size. If I can be ant help, let me know. One point, the restriction must not be abrupt. the pressure can be in the reducer, at a known dia.

Rob