Variable Displacement Piston Pump Failure Analysis How it works and How to Repair

This Weeks Video features a Nachi Twin Discharge Variable Displacement Piston Pump removed from a Hitachi EX40 Excavator.
We show the cause of the failure and the subsequent damage caused.

We briefly describe How a Variable Displacement Piston Pump Works or operates and we then show you the piston pump assembly procedure for a typical Piston Pump Repair and overhaul using new parts including drive shaft, bearings, complete rotating group (Barrel and pistons) as well as new hardened Steel Swash Plate and Bronze Alloy Valve Plate.

Time Stamps
0:00 Introduction
0:45 The Cause of the Damage
2:00 Change of Displacement Explained
2:55 Twin Discharge Barrel Explained
5:05 Time-Lapse of Assembly
10:20 Assembly Complete.

For a better explanation of suction and pressure generation of piston pumps please see
   • Swash Plate Piston Pump (axial flow v...  

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The machine's owner contacted us and advised us that the excavator had lost power was very noisy and had a bad vibration. So before removing the pump we decided to travel out to site and perform a flow and pressure test on the pump.

The pump's two outlet ports only generated 10 and 15 liters per minute respectively and could not generate more than 500 PSI (35 bar) pressure from each discharge port.

Based on the flow test results we decided to remove the pump from the machine and bring it back to the shop for a strip down and investigation.

After dismantling and stripping down the Variable Displacement Piston Pump we discovered that the ball end of one of the 10 Pistons had failed and had broken off and dropped the bronze alloy piston slipper into the pump's case.

This caused some of the broken parts to be sucked in by the rest of the pistons causing abrasion and wear to the remaining piston slippers valve plate and swashplate as well an imbalance to the piston barrel (rotating assembly) creating vibrations.

Dylan Loo Maurelli, for Video and Editing

Variable Displacement Piston Pump a Hitachi EX40 Excavator.
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