# Measureing Output Power



## Captain Jerry (Jan 7, 2009)

No shop time this week. I'm driving to Ft. Gordon, GA to see my grandson, a new U.S.Army 2nd Lt., graduate from Communications School. Last school before deployment.

So I'm thinking..... How could I test the power output or torque of a small air/steam engine. I'm not satisfied with Finger Loads or "ought to be able to do real work" estimates. I want real numbers. Torque in Inch Pounds or whatever you metric people use (gram-mm?). Something for comparison. Not just for bragging rights but for testing the effectiveness of modifications or tweaks.

So far the best I have come up with requires only a pair of digital fishing scales and a strap. With no other equipment and without any calibration required, this device should be able to give an accurate measurement of torque at stall. If you add a tachometer to the mix, you should be able to measure torque at any target speed.

Here's how it works:

Connect the two scales with the strap. When you pull the two scales apart, they should both read the same value...the force that you are applying.

Now attach one of the scales to a fixed point a the same height as the top of the engine flywheel. Lay the strap over the flywheel opposite the direction of rotation. Let the other scale hang from the strap. The flywheel should be able to turn freely. Note the difference of reading of the two scales. It should be very small. This is the only callibration necessary.

Now with the engine running, slowly apply downward force on the hanging scale until the engine stalls. Note the difference in the two scale readings. Subtract the callibration value from above and divide by the radius of the flywheel. 

The results is inch-pounds of torque at stall. See the attached for graphic of setup. In this example, the difference is 2 pounds (ignoring the callibration value) and the flywheel has a 1 inch radius so the stall torque is 2 inch pounds.

I'm just thinking. No real testing. Does it sound right?

Jerry 

View attachment Torque Test.pdf


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## T70MkIII (Jan 7, 2009)

Would a lower-tech variation be to use a string around flywheel of known diameter to lift a bucket, where you add weight to the bucket until it stalls?


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## Brass_Machine (Jan 8, 2009)

Dunno if these will help. Here are a couple of links on output power of model engines. Didn't read them through... so I am not 100% sure if it will help. 

Testing Model engine HP

Model Dyno

Eric


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## Captain Jerry (Jan 8, 2009)

T70MkIII  said:
			
		

> Would a lower-tech variation be to use a string around flywheel of known diameter to lift a bucket, where you add weight to the bucket until it stalls?



I'm not sure. Do you mean to wrap the string around the flywheel and wind the bucket up? If so, you will have to get enough weight in the bucket to stall the engine before the bucket gets to the flywheel.

If you mean to apply friction to the flywheel as in my drawing, you need the difference between the tension on the top and bottom scale to know how much of the weight is being absorbed by the engine.

I think you need the reading from the top scale. I think, but I could be wrong. NO, the more I think about it I'm not wrong. But then..........?

Jerry


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## Captain Jerry (Jan 8, 2009)

Brass_Machine  said:
			
		

> Dunno if these will help. Here are a couple of links on output power of model engines. Didn't read them through... so I am not 100% sure if it will help.
> 
> Testing Model engine HP
> 
> ...



I read both and they were way more complicated. Probably more accurate too but they both seemed to be aimed at testing hi-reving IC engines that have relatively low torque at low rpms leading to stall. Steam engines produce high torque at low RPM.

My approach may be all wrong. That's why I'm asking the question.

Jerry


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## dparker (Jan 8, 2009)

Hello All: The measurement of power can be done with a Prony Brake, several examples are available to look at if you use Google. If I remember correctly, Strickly IC magazine had a fairly exotic one in a "build it" set of articles several years back.

My Dad built one to test the power of a couple of surplus motors he rewound for making his electric car back in the late 1970's out of a 1954 Metropolitan. We did tests on the motors before and after he did the rewinding of the stators. The prony brake was mounted to a scales on a arm of known length from the centerline of the motor shaft and the brake was tightened around the shaft by a bolt squeezing the wood "brake pads" together on the shaft. The speed was measured with a old Starrett hand held tachometer and a watch second hand to compute the speed. (had to be fast or we got a lot of smoke from the wood).  As I remember we got some fairly good charts made of the differences of power with different voltages to the motors. 

I built a much larger one to calibrate the electric motors in the Test Lab that was a swinging cradle on which the motor sat with the shaft going through the center of rotation out to a pump to absorb the power. There was a setup for adding calibrated weights at a known distance from motor centerline and a machinist's level to bring the platform back to the same position for each reading. The speed was measured with a digital photo tachometer. The power absorbed was varied by changing the flow on the pump. This gave us RPM and torque from which we could determine output horsepower output. We were able to calibrate electric motors up to 300 horsepower with this unit that satisfied all the ISO requirements to be able to test pumps and sell them with certified curves all over the world.

The big problem is to be able to load the motor with a constant load for long enough to get the other readings and be able to do the computations to make a graph in whatever style or units desired.
don


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## shred (Jan 8, 2009)

I'm thinking a single-cylinder steam engine probably has an odd looking power curve, so finding out where it stalls may not be so useful. I'm thinking use it to drive a generator and increase the load until it gets unhappy, then work from there.


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## Kludge (Jan 8, 2009)

One of the approaches I considered for my Ryobi engines was in two parts, a fishing scale for thrust and an arm of known length pressing on something like a postal scale which I _think_ should give a result in foot pounds. That assumes a dragless carriage for the engine itself so that both readings wouuld be fairly close to accurate but I'm not entirely sure that the torque would read right since that would be affected by the prop installed. I think. 

On the other appendage to be identified at a later date, a set of relative measurements would be possible for various propellors on the same engine over a range of operating speeds which, to me, is more important. Also, with a given prop, I can measure the results of fiddling with the engine including (by that time) inlet air temp, CHT, EGT, manifold pressure (4-stroke, such as it is) and so on ad nauseum ... including as someone mentioned alcohol injection. 

In this case, given that simplicity is desired, I think Jerry's response is a good start. 

BTW, Jerry: From a vet to your son: Congrats and best wishes in your career.

BEst regards,

Kludge


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## artrans (Jan 8, 2009)

I could be wrong but I think that it shows the stall speed of the motor and not power.This reason I say that is thats at a extrem load. In the case of a airplane thrust is what we want,so why not mount motor to a dolly and use the scale to measure pounds of thrust at full power that will show you how many pounds of thrust I think.


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## Captain Jerry (Jan 8, 2009)

Kludge  said:
			
		

> Also, with a given prop, I can measure the results of fiddling with the engine including (by that time) inlet air temp, CHT, EGT, manifold pressure (4-stroke, such as it is) and so on ad nauseum ... including as someone mentioned alcohol injection.



I think that's what is most useful. To be able to test the effect of tweaking valve timing etc.

The prony brake suggested by DPARKER sound good too but with the low power involved with very small engines, I'm afraid that the weight of the arm itself would make up a large percentage of the power. I'd really be tickled if I built an engine with enough power st smoke the wooden brakepads. In any case, the weight of the arm divided by half it's length would have to be added to the scale reading.

I agree with SHRED about the odd power curve. I guess a tachometer to check torque/speed across a wider speed range is required. Using a generator/load is good, particularly for comparative test of the same engine but wouldn't you have to have volt and amp meters and know the efficiency of the generator to get real power numbers? Sounds complicated.

As I think more about this, it seems like you would have to include air PSI and volume to get real numbers. PSI is easy, but does anyone know how to measure volume? That would be very useful for testing piston and valve sealing.

Jerry


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## Captain Jerry (Jan 8, 2009)

artrans  said:
			
		

> I could be wrong but I think that it shows the stall speed of the motor and not power.This reason I say that is thats at a extrem load. In the case of a airplane thrust is what we want,so why not mount motor to a dolly and use the scale to measure pounds of thrust at full power that will show you how many pounds of thrust I think.



Isn't the stall speed always zero? Thrust is a good measure of engine/prop combinations if you are flying planes, but what if you are sawing wood, turning brass, or elevating monsters? ;D

Jerry


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## Kermit (Jan 8, 2009)

http://books.google.com/books?id=u4...me+mover&lr=&as_brr=1&as_pt=ALLTYPES#PPA41,M1
 A good treatise on Dynamometers starts on page 41 and goes on for a few pages

Kermit


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## Captain Jerry (Jan 8, 2009)

Kermit

Thanks for that link. I really enjoy reading about steam engineering written when steam was THE major source of power. The book goes into great detail on both block brakes (Prony brake) and flexible brakes like the one I suggested. It seems that all new ideas are old ideas.

Jerry


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## Bernd (Jan 8, 2009)

Jerry,

I'm wondering if the material used to go over the flywheel causing the friction is an important factor. I mean if you use leather as opposed to, say, plastic. The friction between the two materials aplyed to the flywheel would bring different readings on the fish scale. Wouldn't it?

Bernd


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## Captain Jerry (Jan 8, 2009)

Bernd  said:
			
		

> Jerry,
> 
> I'm wondering if the material used to go over the flywheel causing the friction is an important factor. I mean if you use leather as opposed to, say, plastic. The friction between the two materials aplyed to the flywheel would bring different readings on the fish scale. Wouldn't it?
> 
> Bernd



Bernd

I dont think it matters. Thats why you need two scales. The difference between the two readings is the friction absorbed.  My guess is that both readings would be lower with leather than with cloth since the pressure required to generate friction would be less with leather, but I think the differences would be the same.

Jerry


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## potman (Jan 10, 2009)

Electronic scales, as used for cooking, beer making and such, are pretty cheap these days and are quite accurate.

earl...


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## Captain Jerry (Jan 10, 2009)

potman  said:
			
		

> Electronic scales, as used for cooking, beer making and such, are pretty cheap these days and are quite accurate.
> 
> earl...



A beer making scale might lead to a loss of concentration on the engine testing but I'm willing to try. Great Idea!

Jerry


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## Brian Rupnow (Jan 10, 2009)

Okay---Very quickly, and off the top of my head---To make COMPARISON power testing with a steam or air powered device---Make a pulley which attaches to the crankshaft. The pulley will have a center hub which is relatively small in diameter, say for instance 1/2" diameter--and will have two large flanges---say 2" diameter. The distance between the two flanges will be wide enough for a single width of heavy cord. Hub will have a hole thru it to pass the cord thru and knot it. Cord is 30" long and other end is attached to long tension spring, like grandma's screen door.-Start engine, and run it under a known air pressure. String will wind up on pulley, and as it winds up it will stretch the spring. At some point the spring tension will overcome the engine torque and stall the engine. Make a mark where end of spring has stretched to. This establishes a "baseline". All other engines when placed in similar position with same pulley, cord, and string can then be measured against this "baseline" measurement. This method will eliminate intangibles i.e. coefficient of friction.---Brian


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## Captain Jerry (Jan 11, 2009)

Brian

Coefficient of friction is not a factor in either the prony brake or the strap brake. In both methods, the scale measures the torque directly.

Consider this. Make two flywheels of the same diameter, one of steel and one of plastic (delrin or HDPE) . Set up a prony brake with wooden brake pads. Engine, Pressure etc the same in both cases. The torque measured in both cases will be the same.

Because of the different coefficients of friction, the only thing that will change is the amount of pressure (squeeze) on the brake pads. In fact, if you wanted to calibrate the clamp screw on the brake pads, you would be able to calculate the relative coefficients of friction for steel/plastic, but the scale at the end of the arm isn't affected.

Waaayyyy back when I was in school, the "Scientific Method" was defined as:

  1. Observe
  2. Hypothesize
  3. Test
  4. Go back to step 1.

I really enjoy mucking around in step 2 when I am away from the shop but eventually will have to take these theories to 3.

Here are a couple of other questions that come to mind:

If the engine also has another flywheel, not the one to which the brake is applied, would the weight and diameter affect the torque as tested by the brake.  I don't think so. Go to step 1.

How do you "measure", not calculate, the energy stored in a flywheel? 

I may be wrong but that is what step 3 is all about.

Jerry


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## Brian Rupnow (Jan 11, 2009)

Jerry--I am out of my depth on this one----Last night I got thinking about what I had posted, and I believe it has a flaw.--As the string winds onto the pulley and builds up, then the radius arm actually changes with each wrap of string. I believe the premise is correct, except that instead of letting the string build up on top of itself, each wrap of string would have to be around the same shaft diameter to keep the radius arm constant. Your idea should work as well---What you are proposing is a type of prony brake with comparitive scales on it. I will take a few minutes here and try to model what I am talking about.--brian


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## kvom (Jan 11, 2009)

Power is defined as P=W/t or P=Fd/t, where F=force, d=distance, t=time.

Why not arrange the engine to raise a measured weight a measured distance, and time it?


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## Brian Rupnow (Jan 11, 2009)

Okay Kvom---In my attached .jpg, the spring is the weight, it gets stretched over a distance, I suppose you could time it, although if one end of the yellow shaft was direct connected to the end of the crankshaft, (which would be a 1:1 ratio) I don't think timing would be an issue. I think the timing would come into play if a gearbox was involved and there was a ratio other than 1:1---

The problem I envision with lifting a known weight a measurable distance in a given time, is "What if the motor isn't strong enough to begin lifting the known weight?" I think it would be better to start out with the given weight being zero, and then increasing it at a known ammount per inch of travel. (This ends up rather like a tractor pull, where the farther a tractor travels, the greater the pulled weight becomes, untill the tractor stalls out or loses traction.)--In a test rig like I've drawn up there could not be any loss of traction, so only pure power would be measured.
 The part I have trouble getting my head around with lifting a fixed weight, is that if an engine can lift the weight an inch, and everything else remains constant, then in theory the engine should be able to lift the same weight a mile!!!


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## Brian Rupnow (Jan 11, 2009)

Jeez, I'm right full of ideas today. (What this means is that I'm so bored I could squeal like a pig!)--What if---the engine in question turned a vane or rotor of some type, in a container of viscous fluid, something like a heavy oil, or corn syrup, at a constant temperature, (to hold a constant viscosity). The maximum RPM that the motor could reach would be limited by how fast the rotor could spin in the fluid. (Of course, you would have to have an accurate tachometer to measure the real RPM) Any tweaking done to the motor could then be very easily tested for performance gain or loss by checking the RPM output. By doing it this way the motor would never have to reach a "stall" condition. It would be a purely dynamic measurement.


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## steamer (Jan 11, 2009)

brake Horse power

HP = 2 x PI x N x T / 33000 ( units of Horsepower)

PI = 3.14159
N = Revolutions per Minute
T = Torque as measured using the brake in foot pounds

Here's a photo of my launch engine on the Prony Brake. The brake is straped to the flywheel and is pulling up against a piece of fishing line which is wrapped around a pulley and anchored to a digital fish scale. I set the length of the line so that the arm is horizontal which is important. Make sure the brake is balanced also with respect to the flywheel. If it is not you will need to add or subtract the imbalance from the fish scale reading. The bearing blocks are maple. The tension screw works against a stiff spring.






Coefficient of friction is not important and does not enter into the equation.  I lubricated the wood blocks a bit so that the coefficient of friction was stable. That is all that matters.  The brake has two small aluminum blocks either side of the cast iron flywheel that keep the brake from coming off, but thats all they are doing.

The brake arm is 1 foot long as measured from the center of the flywheel to the point where the line attaches to the arm.

Set up that way the fish scale ( reading in pounds and tenths of pounds) reads foot pounds directly.

You will notice a small silver looking square on the flywheel, that was for a digital optical tachometer.  It was very easy to take data. You have to throttle the engine up a little faster than you want to go, then load it with the tension screw back to the rpm your are interested in ( 600 rpm for my engine).  You have to do the reverse lower the power level.  Don't just unload the brake as the engine will run away!. 


It was very easy to run, but I did lubricate the brake as the friction coefficent would change as the dry wood got hot.  A little oil solved that problem. Very Stable.

Dave


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## Captain Jerry (Jan 11, 2009)

Steamer

Thank you for putting it to the TEST. It looks like you have been at this for a while. Can you give us some numbers like bore, stroke, pressure and torque for this engine at various RPMs? I'm just curious.

I am certain of few things but I think it unlikely that at my age, I will have an original idea. I vow never again to say "I just had an idea." It would be better to say "I just remembered something from somewhere." But I do love these discussions and the turns that they take. From power output, (I should have said torque), to beer making and now Brian is hot on the trail of some kind of high-fructose torque converter. ;D ;D ;D Where will it end?

Jerry


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## steamer (Jan 11, 2009)

Hi ya Capt,

The engine is a 1.5 x 3.0 x 2.5" stroke compound.  At the time of this test, I had 80 psig of WET steam.

The engine was designed to run on 160 but the boiler was not built then and I was hooked up to someone elses stationary boiler.

The torque was approximately 6.5 foot pounds with a HP of .75 @ 600 rpm.  With dry steam at full pressure and a vacuum in the condensor, I figure I am between 1.75 and 2.00 BHP @ 600 rpm.  I haven't had her on the brake yet with this much pressure, but the boat goes like a scalded cat on 140 psig.
Steam engines have very flat torque curves.  As a matter of fact they produce maximum torque at ZERO rpm.

Of course...if it isn't turning it is not generating power....just torque.

You could easily produce a small prony brake for demo purposes.  I may for the upcoming NEMES show in Feb. I have a small engine that would be wonderful for demonstrating the concept of power to the younger crowd. Just keep your units straight and the math is easy enough.

If the torque arm is smaller than a foot, divide the length of the arm by 12" and you will have the fraction of a foot

I choose 1 foot just to make the math easy.

Dave


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## Stan (Jan 11, 2009)

In the days of farm steam engines, HP was measured with a fan. I do a little work with a museum and ours is about eight feet in diameter in a shroud about three feet wide. As it sits out in the field (currently covered with snow) I am not going to get all the data off it. If anyone is interested I can probably get it from the curator.


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## steamer (Jan 11, 2009)

I was recently reading an article in model engine builder #15 I think on a inertia dynomometer.

This device was built to test small two stroke race engines that run in the 25000 rpm range.

The engines are usually running either full throttle or idle throttle.  To test performance, they built an inertia brake instead of a absorbtion brake.  The engine accelerates a flywheel and the period over which it is accelerated is proportional to the horsepower produced.  I you accelerate twice as fast you have 4 times the HP.

Nice article and write up.

Dave


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## Brass_Machine (Jan 11, 2009)

Ya know... with the knowledge and talent on this board... this might be a great idea for a team design. We could design a tool to measure power output of small steam engines. It could be scaled up or down depending on what the member wanted to test.

Thoughts?

Eric


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## Philjoe5 (Jan 11, 2009)

Stan,
is the fan you're referring to called a "Bakers Fan". I think the museum at Kinzers, PA has one (or more). There's one in the model building that looks about right for testing model steam engines.

Cheers,
Phil


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## Stan (Jan 12, 2009)

Phil: I can't say for sure but I don't expect there were a large number of manufacturers and it was likely patented. Our's has a nameplate and also a museum write up about it. On demonstration days, we run the big steam tractors on it and then run old diesel tractors as a comparison. Our biggest steam tractor is 125 HP for belt power (driving a grain separator) and we have a 85 HP for drawbar power (pulling plow)


Building road with a small steam tractor


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## Brian Rupnow (Jan 12, 2009)

Baker Fan






In the words of Pat Cahill

The Baker Fan has been in use since the early 1900's. It is a replica of the orginal fan invented by Abner D. Baker used in testing the Baker stream traction engines to determine the belt horsepower. Steam engines were used to supply power to run threshing machines, sawmill and other belt driven machines and also for pulling plows for breaking of sod for farm land.

After seeing a Baker Fan in operation at Irricana, Alberta in 1994 three men, Don Fox, Pat Cahill and George Yorga decided to construct one. A trip to Saskatoon where they have a Baker Fan at the W.D.M.was made where photographs and measurements were taken.

For the main frame oil drillers drill stem was used, the shaft and pulley came from an old Nicholas and Shepherd threshing machine,the fan blades are 24"x24"x1/4"sheet steel. There were many delays while searching for materials to construct the fan.

It was completed in1997-98 and presently sits at the Sukanen Ship Pioneer Village Museum,just to the west of the temporary station.



Resistance is produced by the fan blades moving air,the faster they rotate,creates more resistance and puts a bigger load on the tractor

It is amazing how hard this fan can make a tractor work, truely something to see in operation.


 Site last updated: Jan 15 2005
©Sukanen Ship Pioneer Village and Museum, 2001,2002,2003,2004,2005
website by Gleim Web Design 

_I can see how this would put a load on an engine, but how would you get any measurement of horsepower from it? This is not unlike the method that I suggested in an earlier post, where a fan or rotor would rotate in a container full of fluid of a known viscosity and measurements of peak RPM would be taken and applied to some base line figures. (I think I actually seen one of these run a long time ago---thats probably where my suggestion came from)_


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## Kludge (Jan 12, 2009)

Brian Rupnow  said:
			
		

> _I can see how this would put a load on an engine, but how would you get any measurement of horsepower from it? This is not unlike the method that I suggested in an earlier post, where a fan or rotor would rotate in a container full of fluid of a known viscosity and measurements of peak RPM would be taken and applied to some base line figures. (I think I actually seen one of these run a long time ago---thats probably where my suggestion came from)_



It seems to be the same concept, Brian, only the fluid in this case is air. It's compressable where a liquid isn't which may make a difference in developing the baseline but I'm not sure I can suss it fully to come up with any fancy numbers to go with it. Marv?

Best regards,

Kludge


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## mklotz (Jan 12, 2009)

GIYF - more HP formulae than you can shake a stick at.

http://www.engineeringtoolbox.com/horsepower-d_472.html

Aerodynamics being the black art it is, I wouldn't trust any formula for the HP needed to drive a fan. My guess is that they calibrate against some known standard.


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## steamer (Jan 12, 2009)

Rudy Kouhoupt had a Baker fan model to run his model engines with.  Its in the "Best of Rudy" either volume 1 or 2

Can't remember which....

Dave


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## Stan (Jan 13, 2009)

I did a search for baker fan and apparently there are a lot of entertainers with the name Baker who have fan clubs. After adding the words steam engine I got a lot of info and it appears that Marv has the answer. One site with engineering data says that horsepower available from the engines being tested was not sufficient to drive the fan to cavitation so accurate HP could be calculated when the fan was calibrated to a known horsepower.

I looked at a lot of sites and it seems like all or most of the pictures are of a replica of the Baker fan.

Here is a bit of trivia for those who build model steam tractors.

Steam tractors were built with the drive axle either under the rear of the boiler or else behind the boiler. The first type (called side wheelers) were used for belt power while the latter (called rear wheelers) were for drawbar power.

An elderly gentleman I met many years ago explained to me that if you used a side wheeler as a drawbar tractor you risked bending the boiler if one of the drive wheels lost traction. In Western Canada these large side wheelers were used by contract threshers and were driven from farm to farm towing and powering the threshing machine.


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