Roller Size

[86rocco]

This site's in Swedish It looks like this guy has modified a torque convertor to serve as a water brake? My Swedish is pretty much not existant, can anybody here help out? Or even expand on the idea.

[willeng]

86rocco:

A water brake. fluid coupling, hydrokinetic coupling, torque convertor etc are all suitable for dyno's, a water brake is easier to make & setup though & is better suited.

The main difference between a water brake & a torque convertor, is that with the torque convertor, the drive impeller--outer casing spins on the flywheel & drives the inner impellor, with a water brake the drive impeller spins inside a case & tries to drag the case around with it, which is needed for torque measurements etc.

With a torque convertor the pump from the transmission fills the torque convertor creating drive, for the water brake the water is controlled--via taps or valving & this is how you vary the torque on the engine just by adjusting the fluid level in the brake.
With the water brake you must have an outlet in the case to keep a controlled constant stream of cool water circulating.

Hope this helps.

[CarlJ]

Hello, this is my first post, been lurking for a while.
In the process of gathering materials for a chassis dyno build this winter. It will be used for garden tractors I pull during the summer.
My question is roller size. I have acquired a roller of a useable length but the dia. is only 5".
I have read that the bigger dia. the better, is this worth going with, or would time be better spent looking for a larger dia. roller?

Thanks
CarlJ

[bruce]

What kind of tires are you planning on using on your tractor for testing?

You want the largest contact patch you can get between the tires and roller. I don't think a 5" roller is going to work very well.

[Tony]

Bruce is right, bigger diameter roller, much better traction, but there are other factors to think about.

You need to think about the Rpm of both tractor and roller, tractor tyre diameter, and the torque/Rpm capability of the power absorber attatched to the rollers.

Most ordinary commercial automotive roller dynos all seem to use use fairly small diameter rollers, usually around the eight to ten inch size. But if you think about it, with a car running in top gear, max engine redline rpm might correspond a road speed of perhaps 100mph to 150 mph for most typical types of road car.

Take 120mph as a sort of rough average. Now roller speed will depend on roller diameter. Most eddy current absorbers have a maximum rated safe operating speed of 4,000 Rpm, and that is probably a fairly realistic design speed for the roller bearings and other mechanical components.

So if you figure on 120mph maximum roller surface speed at 4,000 roller Rpm.
That works out to 10,560 feet per minute roller surface speed.
10,560/4,000 = 2.64 feet circumference (or 31.68 inches)
Divide by pi, to come up with almost exactly a ten inch diameter roller.

But many standard unmodified road cars are not going to reach anything like 120mph in top gear, so many commercial tuning dynos often have roller sizes an inch or two smaller than ten inches. But that is the sort of basic thinking that establishes roller size.

How fast does a tractor go absolutely flat out in top gear, I have no idea ? I suspect that the practical gearing of a tractor may require a theoretical optimum roller diameter that is just too small to get sufficient traction. Large rollers may be unavoidable.

One possibility might be to use some really extra large diameter dyno wheels on the tractor with dead smooth tyres, combined with suitably larger diameter dyno rollers. That would reduce tractive effort, and increase roller surface speed which might make non slip power transfer into the rollers more efficient with a low geared tractor.

[rab]

[color=#000000]I have an old roller dyno that I got, it has an 11in roller which looks to be 2in thick wall, on the one end it has large thick disk brake with two callipers they look as if they run on air, then it has an arm out to small hydraulic cylinder, and on the other end it has a ring gear for a starter motor, it came with no gauges or any thing, what I am trying to work out is how or were to hook up a DAQ too, I know one will be the hyd cylinder but were do I set up to get rpm

[racear2865]

You will need to measure the RPM at the roller. Yes the torque and hp is figured the same as an engine dyno. You will have to take into account the inertia of the roller. You will need to know how many samples per second that your data acq card is sampling at. Your sampling needs to be as high as you can afford. Also you need to determine how many channels you will be sampling and exactly what do you want to measure. You cannot control what you do not measure.
It would be a good idea to read engine rpm also so that you can campare the two too see if you are getting tire slippage. Are the rollers smooth or ribbed or knurled. Your biggest challenge will be to stop tire slippage.
racear

[99-1134301289]

hello I put questions with the manufacturer of this bench the answer that it my given here

Hello



Well its some years ago now since I build my dyno, as you can see I chosen an easy way there the house is turning and the turbine is fixed (static) the turbine is connected to a lever which put over the torque to a cylinder which is connected to a pressure gauge and then I measured the force and the length of the lever to get finally the horse power.



The problem I have was to circulate the water because then I started to fill up with water the turbine locked up almost direct I tried to solve the problem through changing the distance between the turbine and the house but even if I had a distance at 25 mm It still locked up (you can see that the house is thicker than original). So what I had to do was to have the engine running at the rpm I should measure and then put on the water, after about 20 seconds the water started to boil.



So if I should build a new one I would let the turbine spin and have house fixed with the measure lever. In that case I could circulate the water even if the converter wasn't filled. (Control the braking force)



Hope this will help you in your coming project



Hans Falkenberg

[CarlJ]

Thank you all for the replys.

Bruce, I was going to use 26x12x12 turf tires as they are close to same size as lawntecs and cepeks.

Tony, IF I reversed engineered your calculations right using my final gear ratio, and possible wheel speed, it figured out to be 5" dia. roller. But furthur investigation does strongly support the use of a much larger roller.
So the search goes on for a larger roller, hate too think what having one made would cost.

CarlJ