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Optical Tachometer:Different Pulley Diameters-QUES

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Andrew H

06-18-2005 16:57:12




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I have a Ford 550 TLB. Proofmeter cable is busted so I can't get a reading on my RPM's. I bought a Laser Optical tachometer on eBay to get a reading on the RPM's. Now here's my question. There are 3 pulleys all connected by one belt. I assume that the true correct reading would be off the main pulley that comes off the motor drive shaft, but I can't access it with the laser. The only pulleys I can access are the water pump pulley and the alternator pulley. Now, the diameters of the water pump pulley and the alternator pulley are much smaller than the main drive pulley. So if I take a reading off them, I am not getting the true engine RPM right?
The water pump pulley is only about 20% smaller than the main drive pulley. Question- IfI get an idle speed reading of 580 RPM at the water pump pulley, what would be a ballpark figure for the engine RPM?

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Jerry/MT

06-20-2005 16:31:41




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
If your rpm is 580 at the water pump pulley and the water pump pulley has a 20 % smaller diameter than the crank shaft then D water pump/ Dcrankshaft = 0.8. Your crankshaft speed is 580/0.8 =725 rpm. The equation is N1 x D1 = N2 x D2 = a constant. So knowing one rpm and the diameter of the respective pulleys, or the diameter ratio of the pullies, you can calculate the other desired rpm



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ejr-IA.

06-19-2005 11:12:45




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
Maybe a inspection plate on the bellhouseing can be taken off and put a tape on the flywheel for measurement.



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Rod F.

06-19-2005 09:13:35




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
Hi Andrew,

First comment I will make is regarding the pulley size. From what I know, the alternator pulley, from factory will be 1/2 the circumference of the crank pulley. The alt should turn at twice the speed of the engine. This is also a reasonably accurate means of measuring speed, as Ford employed this system on the thousand series tractors for 10 years.
Second comment, or more a question, is why you would go through all this bother for no more than 50 dollars worth of parts to repair the factory setup? If you change this, you lose the factory tach and proofmeter, and just add more clutter to the dash panel. Beyond that, you may find that the electronic instruments don't care too much for the vibration, moisture, and dust they will encounter in that environment. That said, I wish you the best of luck with the conversion if you go ahead with it.

Rod

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MarkB_MI

06-19-2005 06:31:17




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
You won't be able to even calculate the ratio between engine and accessory speed for anything that's driven by a V-belt. It's going to vary with belt tension.



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Jerry/MT

06-18-2005 22:45:43




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
Do you have a PTO on this machine? If so, and if you know the gear ratio of the PTO, you can mesure the rpm of the PTO shaft and calculate the engine rpm. Other wise, you need to know the diameter of the crankshaft pulley and the other pulley and then you maeasure the rpm of the other pulley and calculate the rpm of the crankshaft pulley.



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NC Wayne

06-18-2005 18:23:02




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 Re: Optical Tachometer:Different Pulley Diameters- in reply to Andrew H, 06-18-2005 16:57:12  
Your right, a reading off of any of the pulleys but the one connected to the crank is gonna give you a false reading. If a larger pulley is driving a smaller pulley the smaller one is gonna be turning at a greater speed (overdriven)than the driving pulley, and the reverse happens if the driven pulley is larger because it will turn slower. If you can get a fairely accurate measurement of both the driving and driven pulleys then you ought to be able to figure out the speed. In the example you gave the crank should be turning 20% slower than the water pump pulley or 464 RPM. Hope this helps and good luck.

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