The 75 pump (model ZB), commonly used in truck cranes, operates by continuously changing the linear motion of seven pistons within the cylinder bore to draw in and compress hydraulic fluid. Over time, if the plungers experience significant wear, the clearance between the plunger and the cylinder bore increases, leading to irregularities in the internal geometry. This results in a drop in pump pressure, reduced efficiency, and eventually, failure of the unit. However, during repairs, it's often found that many older 75 pumps can still be restored and reused.
After proper repair, these pumps can typically function for another 2 to 3 years, making it a cost-effective alternative to replacement. While some hydraulic components are not interchangeable, certain parts of the 75 pump—particularly the plungers—can sometimes be reconditioned and reinstalled. The cylinder bore is more difficult to repair, but the plungers are easier to restore, allowing the pump to regain its original performance.
There are three common methods for repairing the plungers:
1. **Reusing Plungers from Discarded Pumps**:
By sourcing worn plungers from multiple old 75 pumps, you can match them based on size and reassemble them. Precise measurements using an outside micrometer and an inside dial gauge are essential to check the plunger’s outer diameter, ovality, and the cylinder bore’s diameter and taper. The goal is to achieve a clearance of 0.02 to 0.4 mm. This method allows the repaired pump to last over two years with reliable performance.
2. **Repairing Using Electroplating**:
If no spare pumps are available, the plungers can be measured and then repaired by electroplating to restore their original dimensions. If the coating thickness is too much, it can be ground down to meet specifications. Special care must be taken to avoid plating the ball bearing area on the plunger. Once properly adjusted, the plunger and cylinder bore can continue to function effectively.
3. **Machining and Reconditioning**:
In cases where the plunger has severe wear or surface damage, such as hard scratches, the plunger can be machined back to the correct size. During this process, it's important to leave enough material for grinding and to ensure proper heat treatment, such as quenching and tempering.
From our experience, repairing the 75 pump instead of replacing it is a smart choice. It allows the pump to perform reliably without compromising quality or requiring expensive replacements. This approach not only saves money but also extends the life of valuable hydraulic components.
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