Skip navigation

Careful design and selection can keep your hydraulic system from being a lemon

Careful consideration and selection of hydraulic components can help ensure your machinery runs long and successfully.

During a recent seminar I presented, one of the attendees — a maintenance manager for a large open-cut mining operation — mentioned that he was considering upgrading the filtration on the mine’s fleet of hydraulic mining shovels — to achieve a higher level of fluid cleanliness.

There’s no shortage of documented evidence to suggest that increasing hydraulic fluid cleanliness increases the service life of hydraulic components — all other things equal. Whether such an initiative would yield an acceptable return on the investment required for the machines in question depends on several variables, which I don’t have room to go into here. But this got me thinking about a bigger issue.

The maintenance principles I explain in my books, Insider Secrets to Hydraulics and Preventing Hydraulics Failures, are about equipping people with the knowledge they need today to optimize the reliability and service life of the hydraulic equipment they have right now. And this is totally appropriate. After all, it’s rarely helpful and not very instructive to tell someone what they should have done yesterday.

Even though equipment design and equipment maintenance are often viewed in isolation, the reality is, many aspects of hydraulic machine design have a significant impact on the machine’s operating cost and reliability, and ultimately, its life-of-machine cost.

Over lunch, the same maintenance manager mentioned that his mine was also starting to think about the replacement of its aging fleet of hydraulic shovels. And it occurred to me, the best time to figure out your maintenance and reliability objectives for a piece of hydraulic equipment is before you buy it, as the sidebar box explains.

Consider viscosity, temperature in machine selection

By starting with the end in mind, you get the maintenance and reliability outcomes you desire — before the machine even gets delivered. To say the same thing another way: you avoid buying a lemon.

For example, you specify the contamination control targets you want to achieve based on your reliability objectives for the piece of equipment. Instruct the manufacturer to deliver the machine appropriately equipped to achieve these targets.

Based on the weight and viscosity index of the hydraulic oil you plan to use, you determine the minimum viscosity and, therefore, the maximum temperature at which you want the machine to run. Then you instruct the manufacturer to deliver the machine equipped with the necessary cooling capacity — based on the typical ambient temperatures expected at the machine’s operating location — rather than accepting hydraulic system operating temperatures determined by the machine’s ‘design’ cooling capacity, which is the norm.

If you don’t think the viscosity/ temperature issue is this important, you’re mistaken. Based on my long experience in the hydraulic repair industry, lubrication failure resulting from low fluid viscosity is one of the biggest causes of premature failure in hydraulic components. And we could continue specifying other requirements that have an impact on reliability, such as minimum required tank oil volume, desiccant tank breather, flooded inlet for all hydraulic pumps, no suction strainers on pump intake lines, no depth filters on piston pump and motor case drain lines, and so on.

Begin with the end in mind

To illustrate my point with an example, let’s consider the oil temperature/viscosity issue. Say I am about to purchase a 25-ton hydraulic excavator. And let’s say this machine is fitted with Acme brand hydraulic pumps and motors.

According to the pump manufacturer, optimum performance and service life will be achieved by maintaining oil viscosity in the range of 25 to 36 cSt. I also know that I expect to use an ISO VG68 hydraulic oil in my particular location, and the brand of oil I’m already buying has a viscosity index of 100.

This being the case, the pump manufacturer is telling me (indirectly of course) that if my new excavator runs hotter than 70°C, the performance and service life of the pumps and motors will be less than optimum. Not only that, with 70°C as the maximum operating temperature, the oil will last longer, the seals will last longer, the hoses will last longer and almost every lubricated component in the hydraulic system will last longer.

So being the sophisticated hydraulic equipment user that I am, I say to the OEM before I order the machine: “I expect ambient temperatures at the machine’s location as high as 45°C, and under normal operating conditions (with no abnormal heat load in the system) I require this machine to run no hotter than 70°C. If you deliver it to site and it runs at 90°C (or whatever) on a 45°C day, then I’ll expect you to correct the problem — at your cost.”

I’m not suggesting this is in the interests of the OEM — clearly it’s not. It’s likely to make their life more complicated and cut into their after sale revenue. No, it’s totally in the interests of the guy signing the checks to keep the machine running.

Fortunately for all the OEMs out there, my current view is that very few hydraulic equipment users will approach a new equipment acquisition with this level of sophistication. But they should. The alternative is to take delivery of a one-size-fits-all machine and live with any reliability shortcomings it may have.

So the next time you or the company you work for will purchase hydraulic equipment, begin with the end in mind. Define your maintenance and reliability objectives in advance, and make them an integral part of your equipment selection process. That way, you’ll get the reliability outcomes you desire.

Brendan Casey has more than 20 years experience in the maintenance, repair, and overhaul of mobile and industrial hydraulic equipment. Contact him at [email protected] or visit his website at

Hydraulic system requirements*

A pre-purchase specification will help you avoid buying a lemon. Consider the following advice when specifying and purchasing hydraulic systems.

  1. All hydraulic pumps must have a flooded inlet (head of oil above the pump’s intake).
  2. Hydraulic pumps must not be mounted inside the tank.
  3. Pump intake lines are to be kept as short and straight as possible.
  4. If installed, pump intake line isolation valves are to be full-bore type and must feature interlocks to prevent the pump being started when the valve is closed.
  5. Depth filters or suction strainers are not to be installed on pump inlets.
  6. Depth filters are not to be installed on piston pump or motor case drain lines.
  7. All hydraulic tanks are to have high-temperature alarms installed.
  8. In closed-circuit hydrostatic transmissions, the hightemperature alarm must be installed in the loop.
  9. All hydraulic tanks are to have low-level alarms installed.
  10. Closed-circuit hydrostatic transmissions are to be fitted with a low-charge pressure warning and/or shutdown.
  11. All hydraulic tanks are to have a drain plug installed.
  12. Pump intake penetrations are to be located a minimum of 100 mm (4 in.) above the tank bottom.
  13. Two-pole electric motors must not be used unless otherwise agreed.
  14. Hydraulic tank volume for open circuit systems is to be a minimum of three times combined pump flow per minute, or mean pump flow per minute in the case of variable pumps, plus 10% air cushion.
  15. Dessicant breathers capable of 3 μm absolute filtration are to be installed on all hydraulic tanks.
  16. All tank return penetrations are to be fitted with drop pipes that extend below minimum oil level.
  17. A tee connection and isolation valve must be installed upstream of the return filter — to enable pre-filtering of top-off oil.
  18. An oil sample point must be installed immediately upstream of the return filter.
  19. Installed filtration must achieve and maintain a fluid cleanliness level of [specify] (ISO 4406:1999) and maximum water content of 500 ppm.

*This specification is not necessarily exhaustive and does not include design criteria covered in other standards. It has been written with maximum reliability and maintenance optimization in mind, not minimum, initial capital outlay. These two things are mutually exclusive.