Re: hydraulic question


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Posted by Charlie on October 08, 2000 at 16:59:31:

In Reply to: hydraulic question posted by paul on October 08, 2000 at 08:58:17:

A pump generates flow and pressure; a motor uses flow and pressure to turn respectively into rpms and torque. Sometimes a pump and motor are identically designed, in which cases differences in dimensions (cubic inches per revolution, for example) merely provide a form of gearing. Sometimes they can be switched from one application to the other. As far as sizing: a basic understanding of simple hydrodynamics and the mechanical systems the hydraulic components are attached to (engine, transmission, PTO, and winch for example) is important.
Example: Your engine idles at 600 rpm. Your hydraulic winch has a gear ratio of 30:1. Your PTO output ratio (rpms output divided by engine rpm) is 0.5:1. Therefore, given same displacement pump and motor, winch rpm at idle will be 10 rpm. With a full drum a Braden MU2 or AHSU3 has a diameter of 11" or dircumference of about 2.9', so that's 29 fpm with a full drum and about 10.5 fpm with an empty drum. But will it pull hard enough? Take the worst case scenario with least mechanical advantage, a full drum. Assume we want to ensure at least 10,000 lb line pull. 10,000 lb times 29 fpm = 290,000 ft-lb/min = 8.8 hp. Assuming we don't want more than 2000 psi at this extreme case, divide 290,000 ft-lb/min = 3.48 million in-lb/min by 2000 psi = 1740 cubic inches/min. At 300 rpm, we need a 6 cubic inch pump. We will want an equal sized motor. If we want higher winch speed we will want a smaller motor but risk wimping out.
The engine/transmission/PTO system is for all practical purposes a bottomless supply of power. Frequently the limiting factor is how big of a motor can be installed onto the PTO, spacewise and also pricewise. My personal philosophy is as above, to be able to develop the rated winch line pull with a full drum at a reasonable line pressure of 1500-2000 psi. This may lead to relatively low line speeds and also the capability to break the cable with a mostly empty drum. One ought to err on the side of too big when sizing cable for a hydraulic winch. For example, I'd use 3/8" on even an 8000 lb Braden or Milemarker planetary though an 8000 lb electric has 5/16". And the big worm drive 10-12,000 winches ought to have 7/16" or even 1/2" though 10-12,000 electrics have 3/8". This is because electric winches stall out at about their rated line pull but as shown above it is very easy to exceed winch rating with a hydraulic winch.
Your "shear pin" can be one of the following:
1) cable breakage; 7/16" breaks at 19,800 lb when in good shape, 3/8" at 14,400 lb.
2) The safety bypass valve on the spool valve (control valve from main pressure line to winch motor) can be set at whatever. I have pressure valves just upstream of all my spool valves
3) You could use a smallish shackle to connect the end of the cable to the chain/hook. Carry spares. Drape an old blanket over the cable if possible. A 7/16" shackle has a "working load limit" of 1.5 tons and shackles have a 5:1 safety factor, so it should break at 15,000 lb. A Braden AHSU3 is guaranteed not to break below 22,000 lb and the 7/16" cable will break at almost 20K, so that's about perfect. You shouldn't get line pulls over 1.5 times the weight (and then only when pulling through deep sucky muck) or about 10,000 lb for a Dodge, so you won't get near breaking strngth for 7/16" cable or a Braden worm drive winch unless something gets really messed up.
Very important with hydraulic winches:
Just about the biggest drawback of PTO winches is the safety factor, it takes two experienced people to run one safely. If you have a manual spool valve you will mount it either outside which prevents operation of the truck and protection or inside which prevents observation of the drum and proper respooling. The best way is solenoid controlled spool valves, which are operated by the same controllers electric winches have. So you can operate it from either inside or outside with a 10-15' control cable. Expensive, but worth it if you can afford it. You then have a winch system with all the attributes of electric and PTO but the drawbacks of neither.
Finally, you will hear that you need huge hydraulic reservoirs. there's a hydraulic winch article in the latest issue of Offroad onto a F250. It's for a relatively modest Ramsey planetary. They claim your reservoir should be 2.5 times your gpm rating, so they recommended 30 gallons!
I have about 3 gallons on my F350 and 5 on my M37. Sure, if you plan on continuous operations like I don't know what you should follow the guidelines. But in all my experience of for example winching across 20-30' of muck (which incidentally is FASTER even in a worm drive hydraulic winch than one of these "fast" electric winches because the electrics slow down to 2-5 fpm at 10-12K and the hydraulic plods along at its' 6-10 fpm, unless you use the accelerator to speed up the motor) the hydraulic oil has never gotten over maybe 120 degrees F.
Charlie


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