Hydraulic Rebuilds Part II

Brendan Casey

In the November-December 2006 issue of Machinery Lubrication, I discussed some aspects to consider when selecting a supplier for hydraulic component rebuilds. Regardless of deciding whether to use the machine dealer, the hydraulic component manufacturer or an independent for hydraulic repairs, their facilities should always be inspected first.

Beyond the basics such as a clean, dry environment, the repairer should have the necessary facilities to carry out the majority of work in-house and test the component once it has been rebuilt.

If the repair shop must rely on outside suppliers to carry out a large portion of the rebuild process, you will likely end up paying too much for the repair. This is due to the margins of these suppliers being built into the price for the rebuild. For example, if a cylinder is sent to a repair shop that uses outside suppliers for welding, machining, rod straightening or chroming, the repair price is likely higher than a repair shop with the capability of carrying out these processes in-house.


It is also essential to ensure the repairer has the capability to function-test the component once it has been rebuilt. This ensures the component will not only work when fitted to the machine, but will also perform within its design parameters.

This is particularly important with pumps and motors. It is a common misconception that the only thing required to return these components to an "as new" condition is to fit a new rotating group. In addition, the rotating group must also be correctly toleranced during assembly so the volumetric efficiency equals or exceeds that of a new unit. The only way to confirm this is to dynamically test the unit under load on a test bench designed for this purpose.

In the case of variable-displacement pumps and motors, the unit's controls must be adjusted according to the manufacturer's specifications to ensure optimum performance and machine productivity. These controls are commonly integral to the unit or mounted on it. This step is often overlooked because many repair shops lack the necessary test facilities or expertise to carry out these adjustments.

Although these adjustments can be made in the field after the rebuilt component has been fitted to the machine, assuming the necessary equipment and expertise are available, it is easier for them to be carried out in a controlled environment. Therefore, ensure the repair shop has the necessary facilities and expertise to test specific components and set its controls before it is dispatched.

When repairs are performed on equipment, it goes without saying that "you should get the job you pay for". It should come as no surprise that, like most industries, the hydraulic repair business has an occasional odd, shady operator. In my experience, these operators are in a small minority and don't survive long, however that's little consolation for those who happen to get caught by one. Here are some traps to look out for when having hydraulic components repaired, and how to avoid them.

Reworking Parts and Charging for New Ones
When a hydraulic component is rebuilt, some parts can be successfully reused after they have been reworked using processes such as machining, honing, lapping and hard-chrome plating. These money-saving repair techniques are advantageous to the customer, as long as he is not charged the price of a new part.

Charging for Falsely Reworking Parts
An example of this is the rechroming of cylinder rods. The repair shop suggests a cylinder rod needs to be rechromed when it knows very well that the scratches will polish out. The customer will often pay for a complete rechrome when all that is accomplished is polishing the existing chrome so it looks like new! This is easy money for the repair shop - at the customer's expense.

When a repair shop quotes rechroming a cylinder rod, always ask if the scratches can be removed by polishing. This indicates to the repairer that you are aware a cheaper solution may be available. In many cases, the rod will actually need to be rechromed.

As a guide, if the scratches are deep enough to catch a fingernail, they are usually too deep to polish out. Polishing the chrome reduces the finished diameter of the rod. This increases the extrusion gap of the rod seal (the gap between the rod and the land of the seal groove), which reduces the service life of the rod seal.

As a customer, the only way to confirm that the chrome job paid for was received is to check the chrome thickness before and after the cylinder is repaired. To do this, use a coating-thickness gauge. Before a rod is rechromed, the existing chrome must be ground off. Each time a cylinder rod is ground, the diameter of the parent metal is reduced and therefore, the thickness of the chrome required to finish the rod to its specified diameter increases.

This means if the rod has been rechromed, the chrome thickness should have increased. If the rod has only been polished, then the chrome thickness (and the rod's finished diameter) will have decreased.

Fitting Used Parts and Charging for New Ones
It is sometimes necessary to rebuild a component using one or more used parts. This situation may arise if the component is an old model no longer supported by the manufacturer or if the required parts are not readily available and it is necessary for the machine to be immediately restored to service. Again, there is nothing wrong with this, as long as the used parts are in serviceable condition, and the customer is notified they are being fitted and are not being charged the price of new parts.

Fitting Nongenuine Parts and Charging for Genuine
As I discussed in my column in the November 2006 issue, there is nothing wrong with a repair shop using nongenuine spare parts when rebuilding a hydraulic component, provided the repairer guarantees the quality of the parts and is not trying to pass them off as new.

Getting the Rebuild You Paid For
There are three things you can do to ensure you get what you pay for. First, find out exactly what you are supposed to be paying for. To do this, ask the repair shop for a component inspection report, which details:

  • parts that are to be reworked

  • parts that are to be replaced

  • the quality of each of the parts being replaced (genuine, nongenuine or used)

Any reputable hydraulic repair shop should have no problems with providing this information to its customers. It is also a valuable reference when conducting failure analysis if the component fails during its warranty period or prior to achieving its expected service life.

Secondly, once this report has been received, and before giving the go ahead to proceed with the repair, arrange to inspect the disassembled component with a representative from the repair shop. Ask the representative to show and confirm the parts being reworked or replaced and request an explanation of why this is necessary. It is essential to be guided by the repair shop's expertise; that is why the hydraulic component is there in the first place. However, if any of the repair shop's explanations regarding the need to rework or replace certain parts do not make sense, get a second repair quote.

Finally, ask for your old parts back. This is a simple and effective way of ensuring the parts that have been paid for have actually been replaced.

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About the Author

Brendan Casey has more than 20 years experience in the maintenance, repair and overhaul of mobile and industrial equipment. For more information on reducing the operating cost and increasing the...