3rd function kit

LD1 · Feb 24, 2017

The crossover relief is commonly used two places.
1. On motors
2. On snowplows.

When you have two hydraulic hoses, like a motor or a plow with a pair of single acting cylinders has, it is basically a double relief that dumps pressure from one hose to the other.

In the case of a hydraulic motor, whatever you are spinning has rotational inertia, and when you stop powering it, the momentum drives the motor until it stops. This action basically turns a motor into a pump.

A motor spool basically opens both ports (both hoses feeding the motor) back to the tank. A spool designed for a cylinder blocks both ports so fluid cannot escape. Otherwise Everytime you let off the stick, your cylinders would bleed down.

So on a motor, if it is hooked to a cylinder spool, blocks both ports and the inertia driving the motor makes it act like a pump, only the fluid has no where to go. The result is either a very sudden stop of whatever the motor was driving, or an extreme overpressure of one of the hoses and likely a blown hose.

What a crossover does (and is usually adjustable) is once pressure exceeds a certain pressure, it opens and dumps to the other hose. So basically like a pump with the inlet and outlet connected together. Can't build extreme pressure that way.

lt190b · Mar 6, 2017

LD1 said:
The crossover relief is commonly used two places.
1. On motors
2. On snowplows.

When you have two hydraulic hoses, like a motor or a plow with a pair of single acting cylinders has, it is basically a double relief that dumps pressure from one hose to the other.

In the case of a hydraulic motor, whatever you are spinning has rotational inertia, and when you stop powering it, the momentum drives the motor until it stops. This action basically turns a motor into a pump.

A motor spool basically opens both ports (both hoses feeding the motor) back to the tank. A spool designed for a cylinder blocks both ports so fluid cannot escape. Otherwise Everytime you let off the stick, your cylinders would bleed down.

So on a motor, if it is hooked to a cylinder spool, blocks both ports and the inertia driving the motor makes it act like a pump, only the fluid has no where to go. The result is either a very sudden stop of whatever the motor was driving, or an extreme overpressure of one of the hoses and likely a blown hose.

What a crossover does (and is usually adjustable) is once pressure exceeds a certain pressure, it opens and dumps to the other hose. So basically like a pump with the inlet and outlet connected together. Can't build extreme pressure that way.

Thanks LD1, for the explanation of the crossover relief in regards to a motor. I think I have that covered with a 4 way/3 position open center, all ports open in neutral valve. These valves and valve plates are available in DO3 and DO5. The PB line on my tractor where I would like to tap, has a short hose in it between the loader control valve and the hydraulic manifold. This hose is a 16mm ID which I think is a #10 or 5/8". Should I use the DO3 or DO5 valve and subplate? This IS for a hydraulic motor, not just a cylinder. Advice welcome.

LD1 · Mar 7, 2017

If all ports indeed open to tank in neutral, yes that would work for a motor just fine.

What machine is this going on? More importantly, how many GPM do your hydraulics produce? -10 (5/8) is typically good for 15-20 GPM. but simply basing it on what is already there isnt the correct approach.

A D03 subplate can handle up to 25 GPM. D05 up to 40GPM. But you have to liik at the valve selection. Just because the subplate can handle 25gpm, doesnt mean that every d03 valve is also rated for that.

lt190b · Mar 7, 2017

The GPM of the tractor is 9.8 . The DO3 solenoid operated directional control valve is listed as: 5075 max psi, 26.4 max GPM, 3045 max psi on tank core. The only spec I found on the DO3 subplate was #6 SAE ports. The DO5 valve is listed as: 4570 max psi, 31.7 gpm, 2320 max psi on tank core. The DO5 subplate has #8 SAE ports. My concern with the DO3 subplate and valve was/is the size difference in the PB line #10 and the #6 of the subplate. Would it be wasteful and unnecessary to use the DO5 valve and subplate?