The 75 pump (model ZB), commonly used in truck cranes, operates by continuously changing the position of seven pistons within the cylinder bore, allowing it to draw in and compress hydraulic fluid. However, when the plungers experience significant wear, the clearance between the plungers and the cylinder bore increases, leading to irregularities in the internal dimensions. This results in a drop in pump pressure, reduced efficiency, and, in severe cases, complete failure of the pump.
During maintenance, many older 75 pumps can still be repaired rather than replaced. After proper repair, these pumps can typically be reused for 2 to 3 more years. While some may argue that repairs are unnecessary, in practice, certain components—like the plunger—can be reconditioned and reinstalled, restoring the pump’s functionality.
Hydraulic components are generally not interchangeable due to tight tolerances, but in some cases, especially with the 75 pump model, it is feasible to replace or repair the plunger. The cylinder bore is more difficult to repair, so focusing on the plunger is often the most effective solution. There are three common methods for repairing the plunger:
1. **Reusing Plungers from Discarded Pumps**:
Look for worn plungers (in groups of seven) from old 75 pumps. Measure the outer diameter of the plungers and the inner diameter of the cylinder bore using an outside micrometer and an inside dial gauge. Ensure that the clearance between the plunger and the bore meets the required range of 0.02–0.4 mm. Once properly matched, the plungers are ground to fit. This method allows the pump to function effectively for over two years.
2. **Repairing with Electroplating**:
If no spare plungers are available, measure the outer diameter of the plunger and the inner diameter of the cylinder bore. If there is significant wear, electroplating can restore the plunger to its original size. Be careful not to plate the area where the ball bearing is located. After plating, the plunger is refined to meet the required specifications. This method allows the pump to continue operating efficiently.
3. **Machining and Reconditioning**:
In cases of severe wear, especially if the plunger has hard scratches or damage, the plunger can be machined and reconditioned. During this process, ensure sufficient grinding allowance is left and that the material undergoes proper heat treatment (such as quenching and tempering).
Based on our experience, repairing these expensive hydraulic components like the 75 pump is far more cost-effective than replacing them. With proper maintenance and repair, the pump can continue to perform reliably without any loss in performance or need for full replacement.
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