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When should a forklift overpressure valve open up?

Martin P

Titanium
Joined
Aug 12, 2004
Location
Germany in the middle towards the left
I bought this cute little antique Datsun/NISSAN FG103 (1.5 ton) in somewhat ragged shape (the social trend is moving towards a secondary forklift) . I pretty much fixed all and am now in the testing/play phase, which quickly moved to picking up increasingly heavy stuff.
I was initially not able to make the forklift raise its rear wheels. I found that surprising.
Even when I picked up two stacked crates of rock, the forklift would raise the crates with the tilt back cylinders, but would then refuse to lift them with the vertical cylinder. Instead of raising its hind legs, the overpressure valve opened up. So it is standing there with the crates 2 inches off the ground, will not go higher, but will not tip either. I feel like I could get another 300 pound out of it before tipping and will probably tweak the valve.
Is this the way it is supposed to be? Possibly for my safety somehow?
I only managed to get the wheels up by putting the fork tips under my primary forklift (weighs 6 tons).

Also, when I move to a limit in either direction of the mast (up, forward, back), even unloaded, the engine will stop immediately. While this makes sense, my large forklift does not do that. Its almost like there is a by-pass valve at the cylinder-stops. I presume this is normal ....?

Any comments appreciated.

Martin
 
Some smaller forklifts will stall the hyd before raising the rear wheels. That being said, you can probably adjust the relief valve a little but I'd use a gauge and not go over 2,500 psi or so.

I have no idea where the relief valve is located on this particular lift. It could be two separate ones, one for the lift and one for the tilt. That would explain why it would tilt more than it will lift.
 
Its almost like there is a by-pass valve at the cylinder-stops. I presume this is normal ....?

I would like to think that it is one in the same as your "over pressure" valve.


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Perhaps the parts bin had a counterweight from a larger model, so the designers went with that?

Rocking the mast is not the same mechanical advantage as pushing the load straight up.

Unless the engine RPM is correct, a low RPM would tend the engine to stalling.

Back to the parts bin theme: pump may be different, cylinders could be smaller diameter, relief valve may be correct, but not adjusted for specific machine.
 
You didn't say where the load was located relative to the load center listed on the data plate. I don't believe a forklift should lift its rear wheels when picking a rated load at the rated load center. But picking the same load at 25% further out on the forks than rated load center should pick the wheels. I have seen the US specs on these numbers, but don't recall them.

Every lift I've had above capacity would curl more than the mast would lift. This can get exciting when you curl to pick a load off a semi and then the forklift gets nose heavy as you try to set the load down.
 
You didn't say where the load was located relative to the load center listed on the data plate. I don't believe a forklift should lift its rear wheels when picking a rated load at the rated load center. But picking the same load at 25% further out on the forks than rated load center should pick the wheels. I have seen the US specs on these numbers, but don't recall them.

Every lift I've had above capacity would curl more than the mast would lift. This can get exciting when you curl to pick a load off a semi and then the forklift gets nose heavy as you try to set the load down.

Good points. Since I do not have any way of weighing 1.5 tons at the moment it is a good assumption that 1.5 tons will not lift its legs, but also will not lift, if all is at load center. My load was at load center, which is 500 mm (20 inches).
I need to look out for pallets of bricks, those would be easy to calculate for weight. I think I have reserves.

I do have a repair manual for the machine. It has one pressure relief valve that is adjusted for a certain pressure. I need to fit a gauge to check.

Learned the term "curl". Good one.
 
Tilt back is usually stronger than lift when the forks are close to the ground. That's where the tilt cylinders have the greatest mechanical advantage. As you get up past the lower mast attachment point, the tilt starts loosing its mechanical advantage.

One time years ago I was lifting a piece 15' in the air, right at forklift capacity, and had to hold it there awhile. The old lift tended to leak out on the tilt cylinders so you had to watch it and tilt back every now and then to keep it into position. This wasn't normally more than annoying but this time, even though it would tilt the load fine close to the ground, that high up it didn't want to tilt it. There I was, lift slowly tilting out, 15' in the air, with no way to tilt it back. Things got pretty exiting and thought we may loose it, until we put a come-a-long from the mast to the counterweight to hold everything until we could set it down.
 
Under no circumstances should your steering axles tires lift off the ground during a full rated capacity lift.
All professionally built fork lifts have lift hydraulic cylinders rated for the tonnage rating of the unit.
This tonnage rating is generally located 24 inches out from the radius of the fork.
With the pressure relief valve is set at the correct pressure that valve should open when the hydraulic lift cylinder see's it's maximum working pressure.
Imagine a lift cylinder on a large fork lift it's a 4 inch I.D cylinder at 2500 p.s.i this cylinder will lift approximately 15.71 tons , now a 2 inch I.D cylinder at 2500 p.s.i (same pressure) will lift approximately 4 tons. This is a crude example however it gives a person an idea of how this works.
The pressure relief is there to protect the hydraulic system from over pressure.
With a proper high pressure test gauge generally able to read up to 4000 p.s.i. you could check to see that your relief is set for the correct pressure. You will need to find the correct working/ relief pressure settings for your model fork lift. Most but not all fork lifts have a test port for the test gauge when setting relief valves.
In short the pressure relief should internally bleed off once your lift has hit its maximum lift tonnage/pressure.
 








 
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