I was subcontracted to remove a 30 GPM, 4000 PSI hydraulic pump from one plastic injection press and install it into a similar system for the same company.

The pump and prime mover are positioned beside and approximately 12 inches below the bottom of the reservoir, creating a “flooded inlet” design. Besides the ball valve at the tank port, there are no additional restrictions.

The Automotive Lift Repair Tampa Florida customer requested that I fill the pump’s case with oil. I did so but assured him that as long as the hydraulic reservoir was not empty, the pump would maintain an adequate oil level. While pre-filling the pump’s case is generally safe, would forgetting to do so cause damage to the pump?

This is a valid question and deserves detailed consideration. As with many hydraulic-related queries, the answer is: “it depends.”

The “it depends” part comes from various factors to consider. In this scenario, we’re dealing with a piston-type hydraulic pump with a flooded inlet. A flooded inlet means the hydraulic oil level is above the pump’s inlet. 

With a flooded inlet, two key considerations are: First, ensure that the pipe or hose between the tank and the pump’s inlet is free of air. Second, if the pump has a case, confirm it is full of oil.

Automotive Lift Repair Tampa Florida Gear and vane-type pumps don’t have a case, so once you purge the intake line of air, you’re set. Don’t forget to open any isolation valve first to purge the air.

For Automotive Lift Repair Tampa Florida hydraulic piston pumps, there’s a different consideration: Some have a flooded housing or case, meaning the inlet port is connected to or shared with the housing. In these cases, flooding the inlet also floods the housing.

Flooded housing piston pumps are identifiable by their lack of a case drain line, as the case is common with the inlet and hydraulic tank. However, special care is still needed during pre-start. If you’ve replaced a flooded housing pump, the intake line and pump housing will both contain air. When you open the intake line valve, oil floods the inlet and housing, but the air gets compressed in the housing since there’s no case drain line to purge it.

To address this, after opening the isolation valve, carefully “crack” the uppermost plug on the pump’s case to let trapped air escape, ensuring the internal parts aren’t dry at startup.

Automotive Lift Repair Tampa Florida Hydraulic piston pumps without a flooded housing can still fill their case slowly if they have a flooded inlet. The filling occurs gradually through internal clearances between the pistons and their bores. This process can take hours or longer. Therefore, if the pump is replaced and the machine is restarted immediately, manually filling the pump’s case with clean hydraulic oil through the uppermost case drain port is crucial to prevent damage.

In summary, before restarting a hydraulic piston pump, loosen the uppermost connection on the housing or case. If oil flows out, the case is full. If not, you need to fill it manually.

Recently, I was asked to provide a second opinion on the cause of failure for a hydraulic piston pump that failed shortly after being put into service. The client had filed a warranty claim with the manufacturer, which was subsequently denied. The manufacturer attributed the failure to hydraulic oil contamination, citing scoring damage to the valve plate as evidence—see Exhibit 1.

Contamination with hard particles that are similar in size to the clearance between lubricated surfaces can cause three-body abrasion. This process results in scoring and severe wear on the sliding surfaces—refer to Exhibit 2.

In Automotive Lift Repair Tampa Florida xial piston pumps, the cylinder barrel is hydrostatically pressed against the valve plate. The higher the operating pressure, the greater the hydrostatic force maintaining this contact. However, if the operating pressure exceeds design limits or if the valve plate isn’t properly aligned with the cylinder barrel, separation can occur. This loss of the lubricating oil film leads to two-body abrasion when the surfaces come into direct contact.

A key indicator that contamination wasn’t the cause of the valve plate damage is the wear pattern. The scoring (bright areas) is concentrated on the inner and outer edges of the valve plate’s sliding surface—see Exhibit 1. If three-body abrasion were the issue, the damage would be more uniformly spread across the surface, particularly between the pressure kidneys, which would likely show the most severe wear.

The wear pattern on the valve plate suggests two-body abrasion due to uneven contact between the valve plate and cylinder barrel, possibly caused by warping or separation. Examination of the cylinder barrel (Exhibit 3) supports this, showing heaviest scoring at the top right and lightest at the bottom left. Uneven contact was also observed between the valve plate and pump head.

The Automotive Lift Repair Tampa Florida valve plate was flat, but its locating dowel was misaligned, causing the plate to tilt relative to the cylinder barrel. This misalignment led to uneven loading, separation, and two-body abrasion of the surfaces. The true cause of the hydraulic pump failure was improper assembly at the factory, not contamination.

Ensuring that a repaired component performs correctly when installed on the machine—before it leaves the shop—boosts confidence for both the customer and the repairer. This is especially crucial for hydraulic pumps.

They expect the rebuilt pump to come with a test certificate, ideally demonstrating that the pump has been tested to full flow and pressure before shipment.

Traditionally, Automotive Lift Repair Tampa Florida bench-testing a hydraulic pump results in 100% energy loss. This means the pump is loaded with a relief valve, causing all input power (pressure x flow) to be converted into heat. While this heat generation is not ideal, it can be managed. The key limitation is that the amount of power converted to heat cannot exceed the power capacity of the test bench. If the test bench has a capacity of 100 kW or 250 kW, that is the maximum power available for the test pump’s input shaft.