Electric chainsaw motor

Affix something to the chain that sticks out a little. Now make another attachment or hold something close by that will hit it. Like the old clothespin, playing card and bicycle spokes. Turn it on and record the sound. Use something like Audacity to measure the frequency. Viola!

I have a new 3.5hp (I think) "Works" largest 14" chainsaw that I want to use the motor for a project. I need to know the RPM. I've had no luck researching this and I no longer have an RPM meter. I imagine all chainsaws have similar rpm. I need to get the ratios on timing belt sprockets close to optimal.

" If this works, you will all be dazzled by my brilliance! "

Or baffled by your bullshit. Either way I am looking forward to being entertained.

A big, 16" bar, 15A corded electric Makita chainsaw has a no-load chain speed of 2900 ft/min -- 48 fps. I'd use my digital camera in timed multi-shot mode (10 frames/sec., I think) and take a series of photos, and then measure the distance between a marker in two sequential shots. You'd have to do it multiple times to be sure you get two visible, sequential photos with the marker.

Then work backward from the sprocket and chain pitches to get the motor rpm.

(Yes, there must be a simpler way.)

For reference, I saw specs on a Greenworks corded 14.5A saw that said

If this is a cordless saw, all bets are off. As for 3.5 hp...that sounds like Sears or Ridgid horsepower to me -- like 22A, if they're talking gross horsepower.

Remove the chain and take the thing to an RC hobby shop. They will for sure have optical tachs for sale. They will probably check the RPM for free if you ask nicely. Make a constrasting mark on the sprocket before you bring it in. Maybe color half the sproket with a black Sharpie. Eric

Hard to know from your (lack of) description, but I'll offer warning...

My limited experience with Works tools is that they're near the bottom for ruggedness. In general, a cheap hand tool rated at 3.5 HP may deliver that for short bursts, maybe. It's not designed for ANY continuous duty, at any power level.

Again, my limited experience with electric chainsaws and related devices is that the universal motors with brushes fail because the brush overheats and melts the plastic, yes, the brush holders are typically plastic. It seems to be working fine, but the next time you turn it on, the plastic has hardened around the brush and it's stuck. You can often take it apart and free it up. It will work for a while until it freezes up again.

I'd not put a lot of time or $$ into a project based on a Worx motor.

and if it doesn't work...?

My Husqvarna 350 tops out around 12,000 RPM.

I've seen a rating in "electrical horsepower" calculated from how much juice it pulls, probably without considering efficiency or power factor.

The question is what is its RPM when producing the power the load needs. Could you fake the drive with hardware store vee belts and pulleys to find out?

My collection of pulleys that weren't the right size has helped a lot on subsequent projects.

I think that some of them, including Ridgid, don't even go that far. They stall it and measure the current; multiply by the voltage; divide by 750; and claim that as "horsepower." My Ridgid shop vac draws, I think, 10A when it's running. But they claim it has "4.25 [peak] horsepower."

HA-Ha-ha-ho!

The RPM at what point? Under load? Tht's going to be tough unless you can rig up a brake or drive a pump with a restricting valve.

Just driving the chain? A couple of people have proposed some methods.

Under no load? Be careful. If it's a shunt wound or permanent magnet motor, it will be OK. If it's a series wound motor, there is no theoretical maximum RPM and it could be damaged by overspeed.

We should call that "puke" horsepower since the motor is about to toss its magic smoke..

Yeah, that would be a better term for it. This is what Popular Mechanics said about it in a shop-vac comparison. To me, this is unbelievable:

"In addition to being designated by capacity, most wet/dry vacs carry

think that means a vac motor is as powerful as a garden-tiller engine, think again. Like many other consumer-grade tools, the horsepower rating is modified by the word "peak." This indicates the electrical draw at the point where the motor is overloaded and stalls. The upshot? Horsepower ratings aren't the most accurate way to compare wet/dry vacs. Looking at amperage makes more sense."

Well, the motor is physically able to do it, just not for long. Kettering made the automotive electric starter practical by designing for brief peak power, 5 HP from a motor smaller than a 1/2 HP continuous-duty motor.

-jsw

OK, but this isn't real horsepower they're talking about -- even for a minute. They're defining "horsepower" as current times voltage (/747).

But if that's "horsepower, then you could have a really big power resistor that developed 10 horsepower.

That electrical value is "power," but it's just *potential* horsepower. If you measured actual, kinetic horsepower, in terms of torque times rpm, it would be a minute fraction of that. If the motor is stalled, there's no real horsepower at all.

I wonder if Sears started that practice? That was the first place I remember seeing it.

Only one thing I can say for certain - it will NOT be a series motor.

It's called "smoke horsepower"

I thought they measured the peak power at the motor's breakdown torque, just before speed drops.

I have a 5.5HP, 3750W generator that struggles to start a 1/2 HP motor on an air compressor, as long as the tank pressure is low. I can't measure the compressor motor's torque vs RPM curve but it clearly is drawing that much electrical power at startup.

-jsw

On vacuums and compressors it's actually "air horsepower" - and there is a defined specification

water column) / 6,356

Don't know about HP of the "TECO Master" (TECO = T Eato Co) shop vac I picked up for five bux six years ago - the seller let it go because no one was interested since it wasn't wet/dry - he had kept it in the original box with the printed printed sheet "owners manual". When I opened it up for inspection I found the date stamp "Aug.31 1971". My son's evaluation as a shop vac is "IT REALLY SUCKS" I keep it in the laundry room outside the shop door for lint trap cleaning, puppy haircut cleanup and any other use I come up with like disk/belt sander dust removal.