Strategies for maintaining hydraulic systems

Hydraulic systems are used in many industries and perform an integral role in many processes. They are just one of many technologies that users need to contend with in maintaining their plants and equipment.
Apart from the user’s own technology base, hydraulic systems require special expertise in order to provide them with effective care, which can be a mammoth task.

For hydraulic systems, there are a few basic items which, if done, will provide a significant improvement in performance and reliability. These items happen to be the same items that most successful manufacturing organizations regard as prime importance in their maintenance philosophy.

The first and most important item is to take correct care of the hydraulic medium. To appreciate this fully, it is important to realize that most hydraulic components are designed to operate with a hydrostatic bearing between moving parts. This is the method employed to provide a low-friction bearing between surfaces with high relative speeds and long displacements, and to simultaneously seal these areas between high-pressure differentials. For example, the slippers of a swash plate pump could typically have 300 bar pressure differential across the sealing face, run at speeds in excess of 10 m/s, and cover a distance greater than 800 km in a 24-hour day.

By design, these hydrostatic bearings allow oil to flow from the high-pressure side to the low-pressure side. In hydraulic terms, this is referred to as internal leakage. This leakage is an integral part of the component design, allowing heat transfer away from the critical areas and retaining the oil film.

Unfortunately, these bearings are also a source of energy consumption and therefore close attention is given to their design in order to reach the optimum balance between efficiency, reliability and functionality. This results in very close tolerances between the moving parts and hence the condition of the oil plays a decisive role in the efficient and reliable operation of the hydraulic components. Typical tolerances found can be as low as 1.5 microns.

Maintainers of hydraulic systems can only influence the conditions under which the components operate by ensuring the characteristics of the system oil are kept at an optimum level. This is normally monitored using specialist laboratories to analyze oil samples at regular intervals. Although these laboratories provide generic information and can assist in interpretations for specific applications, it is important for users to review the lab report in relation to their systems and assess if the conditions noted in the report are acceptable.

Knowledgeable hydraulic vendors are in the position to assist and assess in setting target levels for the oil condition and offer advice or recommendations on actions required to improve it.

The most important facets of the oil are contamination levels, water content and viscosity index. It is important to review the actual values as well as the change in these values. The actual values should be in accordance with the system manufacturer’s recommendations.

In a well-maintained system, there should be very little or no change in these values. Any dramatic change is an indication of something detrimental happening in the system that may not be visible yet. The cause of the change should be determined and corrected as soon as possible.

Ensuring the oil is kept in optimum condition is of utmost importance and will ensure the maximum life from hydraulic components. Monitoring and maintaining the oil can be a low-cost item resulting in benefits that far outweigh the required cost involved.

Causes of hydraulic component failures

In our repair facility, we very rarely see components in which the hydraulic portion of the component is worn. Almost all components are being repaired as a result of contact seal wear, the result of operation with inferior oil or of having been subjected to abnormal conditions.

Also interesting is that those components coming in for preventive maintenance have little evidence of operating with contaminated oil, whilst oil contamination is present in most cases where the item is brought in for lack of performance.­­
After dealing with the condition of the hydraulic oil, a second area of importance in reducing hydraulic system failures is to assess the technical competencies required to support the product range deployed in the plant and ensure that the users, combined with the logistic support provided by vendors, possess all the skill sets required.

Some hydraulic components are already complex and the trend is for them to become more complex. Hydraulic components can no longer be regarded as pure mechanical mechanisms, as they employ critical design features that need to be maintained. Many hydraulic components today may even include embedded electronics and sensors.
Their accuracy, energy consumption, response time, repeatability, etc., require careful compliance and adjustment to ensure their performance characteristics are achieved. It is, therefore, as a third area of importance, critical for users to have access to a vendor connected to the technology from where these components originate.

Knowledgeable vendors understand the product features and what aspects are important in the user’s specific process. These vendors play an important role in the effective maintenance of the plant and fill gaps in the user’s armour. This is not only required to ensure competence with current technology, but also to ensure one remains connected to the manufacturer in order to realize the full benefit as these components become more complex.

The common guideline here is to establish access to the manufacturer of the brand of equipment used in the plant. This access can be direct with the manufacturer or via an officially appointed agent.

For critical components, the supporting vendor, if indirect contact is made, should be validated. Users should not rely on a vendor’s claim to being an agent. Today, most equipment and spare parts are freely available and any provider with initiative can appear to be connected to the technical support base. In many cases, the source of this secondary support is not legitimate and may be based on opportunistic sales without offering a committed technical support base. Although these vendors may provide a suitable level of support when all is going well, they will not be able to provide support when technical difficulties appear.

A fourth area related to hydraulic system maintenance is to ensure the components are repaired in accordance with the manufacturer’s specifications. Apart from the mechanical nature of the product, there are close tolerances required in the repair process that need to be adhered to or set in order to ensure effective operation. Ignoring these requirements will render the operation less effective and efficient, and repairs will not last long before failure sets in again.

To ensure one does not fall prey to inferior repairs, it is important to assess vendors for their ability to repair a product. This may sound fundamental, but there are many vendors out there professing to be experts who have no or little connection to the technology base from where the products originate. They are therefore not aware of the finer detail required to ensure products are brought back to standard during the repair process.

For the important items, this assessment should include concrete verification of formal agreements with the companies represented, and could even include checking the manufacturer for confirmation and for their assessment of the level of support provided by the vendor.

One particular area of concern throughout support organizations are companies presenting themselves as central purchase houses. In many cases, these purchase houses undertake repair work themselves, rather than having it done by authorized repair houses.

Although these houses provide what appears to be inexpensive repair work, they disconnect the user from the technology base and in many cases are not connected to the technology base themselves. They are therefore not technically competent to detect trends, advise users regarding process improvements, or alert the manufacturer to areas of product improvement.

By the time the manufacturer gets involved, problems are normally well developed, considerable costs have been incurred, and no history exists to solve the problem effectively.

Summary
There are many aspects of hydraulic systems that require attention, but the steps discussed here are offered as a basic approach that will assist in reaching the maximum service life from your components with optimum efficiency and consistency. The key points are:
1) Keep the hydraulic medium in a good condition and attend to the most significant element of effective maintenance of hydraulic systems.
2) Ensure access to all skill sets required to support the technologies employed in the plant.
3) Use qualified vendors to complement individual skill sets and provide access to technology pools, and involve them in all maintenance activities.
4) Ensure that during repair, components are brought back to original design specifications.MRO

Louis Vally is the general manager, hydraulic service, at Bosch Rexroth Canada, Burlington, ON.