Fitting power steering to a tractor: hoses, pipes and which port goes where

A power steering kit for a tractor built with manual steering has a pump, a steering unit (orbitrol), a cylinder, a reservoir with a filter, and the hoses and pipes between them. The unit has four lettered ports: P comes from the pump, T returns to the tank, and L and R go to the two ends of the cylinder.

What parts are in a tractor power steering kit?

A pump, a steering unit, a cylinder, a reservoir with a filter, and the lines between them. This article covers the hydrostatic type, with nothing mechanical left between steering wheel and front wheels. Danfoss, which makes steering units, puts it this way in its technical information:

“… there is no mechanical connection between the steering column and the steered wheels. Instead there are hydraulic pipes and hoses between steering unit and steering cylinder(s).”

Danfoss, Technical Information: General, Steering Components, March 2021

As the wheel turns, the unit meters oil to one side of the cylinder, and the oil pushed out of the other side returns to the tank. In an open-centre system a separate fixed-displacement pump feeds the unit. If the pump stops delivering, the unit acts as a hand pump on its own; Danfoss says that serves only “for a limited control of the vehicle”.

What do the letters P, T, L and R on the steering unit mean?

They name the four ports. The Danfoss service manual for its OSPB, OSPC and OSPF units labels them “P: Pump”, “T: Tank”, “L: Left port” and “R: Right port”. Oil goes to cylinder port L or R “depending on the direction of turn”.

Letters from the Danfoss service manual (March 2014). Another maker’s unit may be lettered differently.

What is inside the unit differs by model. In the Danfoss catalogue the OSPB “has no valve functions”, while the OSPC ON has a check valve in the P port, a relief valve, shock valves and suction valves. The relief valve protects the pump and the steering unit against excessive pressure.

Can the pressure line and the return line use the same hose?

No. The two lines do not see the same pressure. For the Danfoss OSPC ON the maximum steering pressure is 210 bar (3045 psi) and the maximum back pressure on the T side is 40 bar (580 psi). In the code table, OSPC ON units have the relief valve set at 140 or 170 bar and the shock valves at 200 or 225 bar.

The shock valves limit the pressure that an outside force on the wheels can raise in the L and R lines; in that table they are set higher than the relief valve. Read your own unit’s settings from its manual and choose hoses to suit. Reading a pressure rating off the hose is covered in the article on hydraulic hose pressure ratings.

DGUV, the German statutory accident insurance body, notes in an information sheet that hose assemblies are used especially where rigid pipes are not an option. A cylinder that moves with the front axle is such a place.

No hemp and no PTFE tape on the ports. Danfoss: “Packing yarn, teflon, and other unsuitable sealing material must not be used on port adaptors.” It asks for bonded seals, O-rings or steel washers, and for torque it recommends the fitting supplier’s figures. End types are described in the article on fittings and thread types.

What does a wrong connection look like?

With L and R swapped the tractor steers backwards; a system drawing air foams in the tank. Four mistakes, four symptoms:

• L and R swapped. The steering works in reverse. On a tractor forum, a Case 580D owner who had fitted new steering cylinders found the wheels going left when he steered right. The reply: “Yes, switching the hoses will make it steer opposite.” And the cure: “Mark the lines to give yourself a reference.”
• Return line narrow or kinked. With an open-centre unit in neutral, pump and tank are connected, so the pump’s flow leaves through the T port. A narrow or flattened return hose puts back pressure in front of that flow. Danfoss lists no symptom for this; it is our conclusion from how the unit is described.
• The system is drawing air. Danfoss lists the signs: “foam in the tank”, jerky movement of the cylinder, and noise. How a loose connection lets air in is covered in the article on tractor steering line leaks.
• Hose close to a hot or moving part. DGUV’s checklist for the inspection before first start-up wants no twist, no tension from a hose that is too short and no rubbing on edges, and asks for a heat shield against high temperatures. On hoses that move in service it also looks for chafe marks and kinks.

How do you fill and bleed the system on first start?

The Danfoss sequence: oil in through a filter, engine at low speed, then the wheel turned slowly from lock to lock at least five times.

1. Oil. Fill the reservoir through a filter. The kit’s manual says which oil.
2. Engine. Start it and, where possible, run it at its lowest speed. Check which way the pump shaft turns.
3. Bleed screw. If there is one, leave it open until oil comes out without foam.
4. Steering. Turn slowly from lock to lock at least five times.
5. Reservoir. Foam, jerky movement or noise mean there is still air. Top up and carry on.
6. Final check. Look for leaks and correct operation; change the filter after running in if necessary.

On a tractor forum, the owner of a 1995 Massey Ferguson 240 wrote of his steering, “it is like there is a lag”. A new filter element in the pump housing cured it.

In what order should it be connected and checked?

1. Unit and column. According to Danfoss, the steering column must put no axial or radial force on the unit’s input shaft.
2. Cylinder. Ports facing upwards, so that no air pocket is left.
3. Plugs. Leave the plastic plugs in until the hoses and pipes go on, and keep dirt out when you remove them.
4. P and T. Pump outlet to P, tank return to T.
5. L and R. Mark the cylinder hoses, then connect them.
6. Hose routing. Any twist, tension, rubbing or hot part nearby?
7. Oil and air. As above.
8. Direction and turns. Steering wheel right, wheels right. Danfoss recommends 3 to 5 turns from lock to lock. With a single differential (single-rod) cylinder the number of turns differs in each direction; that is not a fault.