How to avoid incorrect selection of hydraulic steering unit?

Avoid misselecting a hydraulic steering unit. A serial number like "101S" may represent a series, and specific parameters can vary between manufacturers, or even within the same manufacturer over time. Misselecting a model incorrectly can not only lead to steering failure but also serious safety incidents. The most dangerous approach is to simply order a new steering gear with the same name simply because it's engraved with "101S." While the correct approach is to use this as a clue, be sure to verify all key parameters listed below.

Table.1: Measures Taken.

1. Engine Information:

● Engine brand, model, and serial number: This is the most accurate source of information.  For example, it should be "XX Brand-956 Loader" rather than just "Loader."

● Engine Year of Production: The steering system may have design changes for the same engine model in different years.

2.  Original Steering Gear Identification:

● OEM Part Number: Look for the full part number directly marked on the old steering gear nameplate or housing by the original manufacturer (e.g., John Deere, Caterpillar) or a first-tier steering gear brand (e.g., Danfoss, Bosch Rexroth, Parker, Bucher).  This is the most reliable basis for a match.

● Manufacturer Make and Model: Confirm the manufacturer and full model number of the steering gear itself, such as "Danfoss OSPC."  Please copy the entire string.

1. Displacement:

● Definition: The volume of oil discharged per revolution of the steering gear (unit: milliliters per revolution or cc/rev). This is the core parameter that determines the steering feel and speed.

● How to confirm: Refer to the host technical manual or the original steering gear sample. The displacement must match the original design, with heavy steering for excessive steering and drifting or too fast steering for insufficient steering.

● Example: The old part has a displacement of 200 ml/rev, and the new part must also have this value.

2. Maximum working pressure:

● Definition: The sustained working pressure (in bar or psi) that a steering gear can withstand. This is related to the security and reliability of the system.

● How to confirm: Also refer to the manual. The rated pressure of the new steering gear shall not be lower than the original system design pressure.

● Example: The system safety valve is set to 170 bar, and the rated pressure of the new steering gear should be at least 180-200 bar or higher, with a safety margin.

3. Installation dimensions and interfaces:

● Installation of flanges: The number of bolt holes, distribution circle diameter (P.C.D.), aperture, and flange shape must be exactly the same. It is best to have a physical comparison or accurate drawings.

● Input shaft: The number of teeth, module, pressure angle, and shaft diameter of the spline or keyway must be completely consistent. This is one of the easiest places to make mistakes.

● Output shaft: The size of the spline or keyway connected to the hanging arm must match.

● Oil ports: The number of oil ports (such as P-pump port, T-return port, L/R-steering cylinder port), size (such as M18x1.5, M22x1.5), type (threaded port, flange port), and position must correspond.

4. Does it have load sensitivity (LS) or pressure compensation function

● Importance: Modern energy-saving steering systems commonly use load sensitive technology. If the original system is load sensitive and you choose a steering gear without this function, the entire system will not function properly and may even damage the pump or other components.

● How to determine: Check if the original steering gear has an additional LS port (usually a small diameter port) and refer to the original system schematic.

5. Does it have a Constant Flow Priority Valve (CFP)

● Importance: In systems that require simultaneous oil supply for steering and other hydraulic functions (such as working devices), the steering gear will integrate a priority valve to ensure priority oil supply for steering under any operating conditions.

● How to determine: When checking the original steering gear oil port, there is usually an "EF" (Excess Flow) or "SF" (Secondary Flow) oil port used to supply excess flow to other circuits.

Provide the information collected in the first two steps to the supplier and engage in effective communication: