Dump trailer hydraulics

[jenkinsph]

Usually if the pump runs in both raise and lower it is double acting, if it lowers quietly with the pump not running it is single acting. On my PJ dump with scissor lift it is double acting this also holds the dump body firmly in place against the frame. I would hope you have a cam lock to hold the bed in place if your cylinder is only a single acting arrangement.

 

[1gr8bldr]

Help me understand why one hose is a typical hydraulic hose, high pressure and the other just a low pressure

 

[bcp]

Help me understand why one hose is a typical hydraulic hose, high pressure and the other just a low pressure

Sounds like a single action system, using the rod end of the cylinder as a reservoir.

Bruce

 

[pmsmechanic]

To me it looks like you have pressure up and gravity down. Whoever plumbed the system put a low pressure hose on the fill port of the reservoir and attached it to the pressure down port of the hydraulic cylinder. Kind of ingenious as the pressure of the cylinder going up helps to pressurize the reservoir and help the hydraulic pump do it's job. If you over fill the reservoir it will just go into the top side of the cylinder instead of dumping on the ground. By the sounds of it you need to add a bit of oil. I would prop up the box so it doesn't come down on you and take off the low pressure hose at the reservoir. Add enough oil so that the pump pressures the hydraulic cylinder all the way to the top of it's cycle lifting up and you should be good.

 

[jenkinsph]

bcp is correct, the fluid is sucked into the rod end as the cylinder is lowered or retracted. This doesn't require a high pressure hose, very similar to a syringe under very low pressure. I take it your pump doesn't run when lowering the dump bed.

 

[Arc weld]

My old 3 ton dump truck hoist used the cylinders as a reservoir but also had a vented cap on the steel hyd. tank. Maybe this system needs a vent in order to fully extend? If it's single acting, the pump shouldn't run when lowering.

 

[jenkinsph]

My old 3 ton dump truck hoist used the cylinders as a reservoir but also had a vented cap on the steel hyd. tank. Maybe this system needs a vent in order to fully extend?


If it's single acting, the pump shouldn't run when lowering.

I mentioned this in post 21 but the OP hasn't responded

 

[timster2]

You may want to check the condition of the lift cylinder. The original owner may have plumbed that line to the tank to allow oil bypassing the piston seals to go back to the tank rather then leak on the ground. That would explain the low pressure line from the rod side to tank. For the piston to act as part of the oil reservoir the line would have to be lower than the oil level in the tank in order to suck the oil back to the rod side when the body is lowered.

 

[1gr8bldr]

Usually if the pump runs in both raise and lower it is double acting, if it lowers quietly with the pump not running it is single acting. On my PJ dump with scissor lift it is double acting this also holds the dump body firmly in place against the frame. I would hope you have a cam lock to hold the bed in place if your cylinder is only a single acting arrangement.

I will listen for the pump on the let down today

 

[1gr8bldr]

I will listen for the pump on the let down today

Motor runs going up but not down. It lifts fast at the beginning, but gets super slow at the top, not quite lifting enough to get the load to slide on it's own.

 

[bd53bz2]

That trailer originally had a single hosed piston. Someone replaced the piston with a dual acting one. The down hose (top of piston) was added and plumbed to what was the vent/fill on the reservoir. You could just remove that added hose and bleed the upper side of the piston in a bucket. The problem with that is that the reservoir will not be large enough to fill the up side on the dual piston. On a single piston you only need enough volume to replace the ram. A dual needs to fill the entire cylinder. To fix it right, you should modify the tank to move the down hose to the bottom of the reservoir, returning the fluid to fluid. Then add a vent/fill cap. Quick fix would be to add a tee on the top of tank and replace the vent/fill cap. The problem with that fix is the fluid will foam. In any case you have to get that reservoir vented again.

 

[jenkinsph]

Motor runs going up but not down. It lifts fast at the beginning, but gets super slow at the top, not quite lifting enough to get the load to slide on it's own.

I find that to be unusual as it is harder to raise the bed from the lowered position (takes more pressure) and as it raises up it requires less pressure to operate. I would suspect it is running out of oil to work with but would first ask if the pump noise changes in any way? Mine is the same going up and down until it reaches end of travel and then loads down.

Timster2 makes a good point about the low pressure hose needs to be mounted lower on the reservoir. As others have mentioned I would wonder if this is the proper tank/pump setup for a large dump cylinder. My Tommy Gates on service trucks use a single acting pump similar to what you posted but the fluid volume to deal with is much less.

 

[1gr8bldr]

I find that to be unusual as it is harder to raise the bed from the lowered position (takes more pressure) and as it raises up it requires less pressure to operate. I would suspect it is running out of oil to work with but would first ask if the pump noise changes in any way? Mine is the same going up and down until it reaches end of travel and then loads down.

Timster2 makes a good point about the low pressure hose needs to be mounted lower on the reservoir. As others have mentioned I would wonder if this is the proper tank/pump setup for a large dump cylinder. My Tommy Gates on service trucks use a single acting pump similar to what you posted but the fluid volume to deal with is much less.

I have never reached end of travel. It used to do better than it does now, as if something changed. The only thing that has changed is me adding fluid after fixing the leak... which was only tightning the top clamp

 

[jenkinsph]

I have never reached end of travel. It used to do better than it does now, as if something changed. The only thing that has changed is me adding fluid after fixing the leak... which was only tightning the top clamp

Can you comment on the pump noises? All of this helps determine the issue at hand. I agree with others that the reservoir needs to be vented and that you need enough fluid available to the pump to fully extend the cylinder. If the pump loads down near the top of travel I would be surprised unless the piston was at the end of travel. I would really expect the pump is running out of fluid in the reservoir and would hear the pump pitch increase with cavitation. Not being there I ask you for this information.

 

[George2615]

Motor runs going up but not down. It lifts fast at the beginning, but gets super slow at the top, not quite lifting enough to get the load to slide on it's own.

Here's some pics of my two systems.
The first two pics are on my tilt bed car trailer. It's a double acting cylinder but I'm only using the bottom port of the cylinder because it is power up and gravity down. The pump doesn't run going down, valve handle just releases pressure and fluid runs back into tank through the pressure hose.
Second two pics are my dump trailer. It also has a double acting cylinder but has a hose on both top and bottom of cylinder since the motor runs for power up and power down.
Since your motor only runs to power up yours is probably wired to release a solenoid when you push the down button. It's the same setup as my car trailer has except the previous owner ran an extra hose from the top cylinder port back to the tank. You should not need the upper hose as fluid would run back through the pressure up hose like mine does once the solenoid releases the pressure.

 

[jenkinsph]

Here's some pics of my two systems.
The first two pics are on my tilt bed car trailer. It's a double acting cylinder but I'm only using the bottom port of the cylinder because it is power up and gravity down. The pump doesn't run going down, valve handle just releases pressure and fluid runs back into tank through the pressure hose.
Second two pics are my dump trailer. It also has a double acting cylinder but has a hose on both top and bottom of cylinder since the motor runs for power up and power down.
Since your motor only runs to power up yours is probably wired to release a solenoid when you push the down button. It's the same setup as my car trailer has except the previous owner ran an extra hose from the top cylinder port back to the tank.




You should not need the upper hose as fluid would run back through the pressure up hose like mine does once the solenoid releases the pressure.

That is fine if your reservoir is large enough to hold all the fluid needed to fill the cylinder. If the reservoir is smaller it won't work without the second hose which can utilize the upper cylinder displacement.

 

[George2615]

That is fine if your reservoir is large enough to hold all the fluid needed to fill the cylinder. If the reservoir is smaller it won't work without the second hose which can utilize the upper cylinder displacement.

As seen in my first pic that is exactly what I did. When I bought the power up / gravity down pump I also bought the larger rectangle reserve tank and replaced the smaller round tank like 1gr8bldr has. After installing and testing mine I found I didn't need the larger reserve tank but wasn't going to switch back. My cylinder doesn't require a long stroke (so less fluid required) to tilt the trailer. For his dump trailer the cylinder has a longer stroke and is probably a larger diameter like my dump trailer. The round reserve tank 1gr8bldr is using is probably too small which is why the previous owner added the extra hose to have more reserve fluid on the top side of the cylinder and if a little low on fluid will create the problem he has not getting full lift. He can buy the larger reserve tank through Graingers or any dealer that sells Haldex pumps and I think that would solve the problem.

 

[jmc]

I've got a dump trailer with power up and power down.

When the reservoir is full, the bed raises until the motor bogs down at full height.

When the reservoir is low, the pump starts to buck and jump when the cylinder is extended enough to run out of fluid.

 

[jenkinsph]

As seen in my first pic that is exactly what I did. When I bought the power up / gravity down pump I also bought the larger rectangle reserve tank and replaced the smaller round tank like 1gr8bldr has. After installing and testing mine I found I didn't need the larger reserve tank but wasn't going to switch back. My cylinder doesn't require a long stroke (so less fluid required) to tilt the trailer. For his dump trailer the cylinder has a longer stroke and is probably a larger diameter like my dump trailer. The round reserve tank 1gr8bldr is using is probably too small which is why the previous owner added the extra hose to have more reserve fluid on the top side of the cylinder and if a little low on fluid will create the problem he has not getting full lift. He can buy the larger reserve tank through Graingers or any dealer that sells Haldex pumps and I think that would solve the problem.

I would agree that if he can retrofit a larger tank or reservoir to his existing pump that this might be a good solution.

 

[1gr8bldr]

OK, thanks for all the responses. This many thinkers and we can figure it out. Update. I raised the bed loaded with trash. As usual, as it nears the contents slide range, before it gets there it just will not go any higher. Pump is pumping as if not under a load. Sometimes I would hold the up button longer trying for 1 more inch. But not understanding the system, I usually don't do that for long because I do not want to burn up the electrics. So I disconnected the hose on top of the reserve tank and added about 1/2 quart, probably much less. Most I spilled on the ground because the air could not be displaced with the oil coming in that 5/16 hole. I was worried about the safety arm since it was loaded so I parked my skidsteer with the forks as a back up. Test run. It starts up strong and fast as ever but quickly starts to slow. This time doing the same thing but at a lower point. Then I let it down. It came down twice as fast as I had ever seen. I could release the down button and it will hold without leak down. But, par for most of my projects, something must have blown because all my fluid came out between the reserve and pump. It is on the bottom side hidden by part of the frame. So, if I did not understand this lift before, I will when this is over. So, do I fix something, throw it away and get a new one or what? I want to plumb everything back the way it should be. Dang, I was needing this trailer next week for a job.