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Old 01-04-2013, 02:38 PM   #10
Enjoying my last V8
fritzcoinc's Avatar
Joined: Apr 2008
Location: Cypress, Tx
Oddometer: 7,288
Originally Posted by DakarNick View Post
Thanks, guys. I want to run two hydraulic motors off a single pump. I'd like the vehicle to run 25mph, which means I would need a pump running 200 rpm if going direct drive.

I found one that is 200 rpm @ 1000psi, 9 gpm, and 1230 lb/ft torque and 10 cubic inch/rev.

So, I would need a pump that delivers 18 gpm. That is quite a bit.

I found a calculator online and figured I could drive the motors with a 10.5 hp engine. Stan, your equation verifies that, also. Just need a pump to mate to the engine.

How does cubic inch displacement work? The motors, combined, would be 20 ci/rev. I'd need a pump that displaces 20 ci/rev?

Or you can connect the motors in series ( control valve port A to motor #1 port A, motor #1 port B to motor #2 port A, motor #2 port B to control valve port B). This way the exhaust from motor 1 drives motor 2 at the same speed as motor 1. This also makes a " posi track " set up. If motor 1 looses traction the flow goes to motor 2. With motors in parallel if one motor looses traction it will spin and rob pressure from the other motors.

If you use a pump that displaces the same as the motor then the " reduction " is 1:1. For more torque use a smaller pump. Exp.: a 5 ci/rev pump and a 20 ci/rev motor gives you a reduction of 4:1. Of course speed is less. The hyd. motor will need some amount of reduction or it will index instead of spining up to speed. Sometimes called stepping.

As to your question regarding the control valve, typically you would use an open center valve with a motor. This valve in the center position connects the pump and both motor ports to tank. The other type of valve blocks both motor ports and connects the pump to tank in the center position. This valve is usually called a tandum center valve. The open center valve prevents the motor from locking up when valve is moved from one direction to the center position. The down side of this valve is there will be no braking. The motor can free wheel with the valve in the center position.

Another way to go is to use the tandum center valve and a crossover relief vavle in the motor lines. The crossover relief connects the lines of the motor together through relief valves. You set the relief valve pressure just high enough to launch the machine. When the control valve is centered and both motor ports are blocked, oil will flow from the exhaust side of the motor to the pressure side through the relief. When the pressure is low enough the relief closes and the motor is locked by the blocked valve ports.This sounds great but in actual pratice adjustment is tricky. Using the open center valve will be smoother but you will need some type of brake.

fritzcoinc screwed with this post 01-04-2013 at 02:47 PM
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