October 15, 2003 

'Inside Hydraulics' Newsletter

Issue 20

1. Revolutionary motor design claims efficiency of 99.9%
2. Solving hydraulic system overheating problems
3. Symptoms of common hydraulic problems and their root causes
4. Content for your web site or e-zine
5. Help us spread the word
6. Tell us what you think


Revolutionary motor design claims efficiency of 99.9%

A new hydraulic motor that was originally developed for marine propulsion applications claims to have overcome many of the limitations of conventional, axial piston motor designs. By eliminating swash plates, sliding shoes and their related components, the Molitech motor design is able to convert the linear displacement of reciprocating pistons into rotational motion of the rotor with an efficiency of 99.9%. Click here to read a detailed description of the motor's design.

2.   Solving hydraulic system overheating problems

I was asked recently to investigate and solve an overheating problem in a mobile application. The system comprised a diesel-hydraulic power unit, which was being used to power a pipe-cutting saw. The saw was designed for sub-sea use and was connected to the hydraulic power unit on the surface via a 710-foot umbilical. The operating requirements for the saw were 24 gpm at 3000 psi.

Why do hydraulic systems overheat?

Heating of hydraulic fluid in operation is caused by inefficiencies. Inefficiencies result in losses of input power, which are converted to heat. A hydraulic system's heat load is equal to the total power lost (PL) through inefficiencies and can be expressed as:

PL total = PL pump + PL valves + PL plumbing + PL actuators

If the total input power lost to heat is greater than the heat dissipated, the system will eventually overheat.

Hydraulic fluid temperature - how hot is 'too hot'?

Hydraulic fluid temperatures above 180F (82C) damage most seal compounds and accelerate degradation of the oil. While the operation of any hydraulic system at temperatures above 180F should be avoided, fluid temperature is too high when viscosity falls below the optimum value for the system's components. This can occur well below 180F, depending on the fluid's viscosity grade.

Maintaining stable fluid temperature

To achieve stable fluid temperature, a hydraulic system's capacity to dissipate heat must exceed its inherent heat load. For example, a system with continuous input power of 100 kW and an efficiency of 80% needs to be capable of dissipating a heat load of at least 20 kW. It's important to note that an increase in heat load or a reduction in a system's capacity to dissipate heat will alter the balance between heat load and dissipation.

Returning to the above example, the hydraulic power unit had a continuous power rating of 37 kW and was fitted with an air-blast heat exchanger. The exchanger was capable of dissipating 10 kW of heat under ambient conditions or 27% of available input power (10/37 x 100 = 27). This is adequate from a design perspective. The performance of all cooling circuit components were operating within design limits.

Pressure drop means heat

At this point it was clear that the overheating problem was being caused by excessive heat load. Concerned about the length of the umbilical, I calculated its pressure drop. The theoretical pressure drop across 710 feet of 3/4" pressure line at 24 gpm is 800 psi. The pressure drop across the same length of 1" return line is 200 psi. The formula for these calculations is available here. The theoretical heat load produced by the pressure drop across the umbilical of 1,000 psi (800 + 200 = 1000) was 10.35 kW. The formula for this calculation is available here.

This meant that the heat load of the umbilical was 0.35 kW more than the heat dissipation capacity of the system's heat exchanger. This, when combined with the system's normal heat load (inefficiencies) was causing the system to overheat.

Beat the heat

There are two ways to solve overheating problems in hydraulic systems:

  • decrease heat load; or
  • increase heat dissipation.

Decreasing heat load is always the preferred option because it increases the efficiency of the system. In the above example, the heat load of the umbilical alone was nearly 30% of available input power, a figure that would normally be considered unacceptable. Decreasing this heat load to an acceptable level would have involved reducing the pressure drop, by replacing the pressure and return lines in the umbilical with larger diameter hoses. The cost of doing this for what was a temporary installation meant that, in this case, the most economical solution was to install additional cooling capacity in the circuit.

Continuing to operate a hydraulic system when the fluid is over-temperature is similar to operating an internal combustion engine with high coolant temperature. Damage is guaranteed. Therefore, whenever a hydraulic system starts to overheat, shut it down, identify the cause and fix it.

"This book has the potential to save many organizations lots of m0ney. It should be on the bookshelf of every engineer, supervisor, planner and technician that deals with hydraulic equipment... it's worth its weight in gold." Find out more

Alexander (Sandy) Dunn
Plant Maintenance Resource Center

3.   Symptoms of common hydraulic problems and their root causes

For some practical advice on detecting conditions that cause hydraulic failures, read Brendan Casey's article in the September-October Issue of Machinery Lubrication magazine, available here. To receive a complimentary subscription to this informative magazine (US and Canada only) go to: http://www.machinerylubrication.com/hydraulic1.asp

4. Content for your web site or e-zine

Need some fresh content for your web site or e-zine? You now have permission to reprint these 'Inside Hydraulics' articles on your web site or in your e-zine, provided:

1. Each article is printed in its full form with no changes.

2. You send an e-mail to editor@hydraulicsupermarket.com to let us know where you'll be publishing them.

3. You include the following credit at the end of each article:
About the Author: Brendan Casey has more than 16 years experience in the maintenance, repair and overhaul of mobile and industrial hydraulic equipment. For more information on reducing the operating cost and increasing the uptime of your hydraulic equipment, visit his web site: http://www.InsiderSecretsToHydraulics.com

5. Help us spread the word

If you've found our 'Inside Hydraulics' newsletter interesting and informative, then chances are you have a friend or colleague who would too. Help spread the word about 'Inside Hydraulics' by forwarding this issue to a friend or colleague. If they share your interest in hydraulics, then they will surely appreciate being told about this newsletter.

New subscribers can join the mailing list by completing the form at http://www.insidersecretstohydraulics.com

6. Tell us what you think

We would love to hear what you think of this issue of our 'Inside Hydraulics' newsletter. And of course, if you have any suggestions for future issues, please send us those too.

Just e-mail the editor at: newslettersuggestions@hydraulicsupermarket.com

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