3 Point Hitch Capacity/Design

[JC-jetro]

Dear friends,

I've got a question and was not able to find the answer. I'd like to seek guidance from the learned folks here as I understand physics but lack actual practical experience with tractor, I reckon I'm moving on up

On my little ole Ford 1700, the hydraulic pump develops 5.3 gpm and 2133 psi @ RPM of 2500. The pump does not have internal relief system and is a gear driven pump driven directly off of crankshaft. At the lift control spool valve (without draft control) I can bypass all fluid to rear differential (sump) or can send it to hydraulic piston to lift. This flow is intercepted first at the system relief valve set up before going to the piston. the relief pressure setting is @ 2133 psi (adjustable). it appears to me I can exert no more pressure to the piston beyond 2133 psi unless the relief valve is stuck in closed position. I don't know how much "dead head" pressure will be if lift is set to raise to the max and relief is stuck. I reckon at that point something got to give, piston seal or something. @ 2133 psi and the area of the piston (unknown) should give certain amount of lbf??

Now my question is; is there any possibility of breaking and damaging upper links of the lift system if relief pressure device is working properly?

What happens if my lift is 1500 LBS and I try to lift 3000 Lbs? do I force the relief to open without any damage or will it damage the piston seals and possibly break the link?

It looks to me if I were to design the link, I would figure out my force developed and made sure the lift design (I.e size, material and design ) would handle 2 or 3 times as much, in other word I would make it "idiot proof"

Much appreciate your input,
Jetro

 

[RickB]

You are pretty much right, a properly set relief valve will open in the case of overload on the hitch. Should happen without any damage to any component unless you let the system run over relief for extended periods which will trash your pump. Shock loads typically cause parts failures, not sustained high loads. Of course we are talking about a 20+ year old tractor and anything can and may happen.

 

[Soundguy]

i agree with rick. On my old 8n, the pump develops 1700psi, and 2.85 gpm, at 2000rpm.

I've backed up to a couple implements that made my tires squat, and then did not lift.. relief int he pump was humming... no harm done.

As rick said.. if you leav in in a relief mode for a while, the oil gets hot.. the pump gets hot, and the relief valve gets hot. After a bit.. that's a bad combination.. in the short term.. it's a fact o' life. I have auto return aux remotes on my NH 7610s. I pull one back to fold the wings on my batwing.. whent he lift cyl deadheads, my relief and detent pop, allowing my remote valve to pop back to neutral.

Soundguy

Dear friends,

I've got a question and was not able to find the answer. I'd like to seek guidance from the learned folks here as I understand physics but lack actual practical experience with tractor, I reckon I'm moving on up

On my little ole Ford 1700, the hydraulic pump develops 5.3 gpm and 2133 psi @ RPM of 2500. The pump does not have internal relief system and is a gear driven pump driven directly off of crankshaft. At the lift control spool valve (without draft control) I can bypass all fluid to rear differential (sump) or can send it to hydraulic piston to lift. This flow is intercepted first at the system relief valve set up before going to the piston. the relief pressure setting is @ 2133 psi (adjustable). it appears to me I can exert no more pressure to the piston beyond 2133 psi unless the relief valve is stuck in closed position. I don't know how much "dead head" pressure will be if lift is set to raise to the max and relief is stuck. I reckon at that point something got to give, piston seal or something. @ 2133 psi and the area of the piston (unknown) should give certain amount of lbf??

Now my question is; is there any possibility of breaking and damaging upper links of the lift system if relief pressure device is working properly?

What happens if my lift is 1500 LBS and I try to lift 3000 Lbs? do I force the relief to open without any damage or will it damage the piston seals and possibly break the link?

It looks to me if I were to design the link, I would figure out my force developed and made sure the lift design (I.e size, material and design ) would handle 2 or 3 times as much, in other word I would make it "idiot proof"

Much appreciate your input,
Jetro

 

[JC-jetro]

You are pretty much right, a properly set relief valve will open in the case of overload on the hitch. Should happen without any damage to any component unless you let the system run over relief for extended periods which will trash your pump.

Thank you Rick and Soundguy. It is good to confirm my suspicion. Additionally the main relief valve on the pump discharge piping has an additional safety device in a form of "fixed spring, not adjustable" check valve that relives the dead head pressure from the lift cylinder. This device is located at the lift cylinder head as the pics below show it. First check valve provide flow to the piston and second ball and spring vent excess pressure through the hole back to sump.

Lift Cylinder Head:


Lift cylinder check valve and relief assembly: