Why Does Hydraulic Oil Overheat?

....because shaft power = volumetric flowrate x pressure change.

In the case of hydraulics, a lot of power is lost at all let-down valves and, because the flowrates in hydraulic systems are small, it appears as a temperature rise in the fluid.

Example:

In a typical marine seawater RO plant, 65 bar is dropped at a pressure-sustaining valve that is passing 30m³/h of water.

Converting the flowrate: 30m³/h = 0.008m³/sec

Assume a density of 1000kg/m³, the mass flowrate is 8kg/sec

The energy conversion is:

65x10⁵N/m² x 30m³/h / 3600sec/h = 53kW

The energy is dissipated as heat, so in an ideal, adiabatic system, the temperature rise in the fluid is typically:

53kW / 4.2kJ/kg.degC / 8kg/sec = 1.6degC _{(as near as damn-it is to swearing)}.

Here's another one.

250 bar is dropped at a pressure relief valve on a water hydraulic system, and 0.5m3/h passes through the valve, spilling back to the water reservoir.

Converting the flowrate:

0.5m3/h = 1.4e-4 m³/sec. At 1000kg/m3, → 0.14kg/sec.

Energy conversion is:

25000000N/m2 x 1.4e-4 m³/sec = 3.5kW.

The energy is dissipated as heat, so in an ideal, adiabatic system, the temperature rise in the fluid is typically:

3.5kW / 4.2kJ/kg.degC / 0.14kg/sec = 6degC _{(near enough)} per pass. In a hydraulic system, the fluid can expect to go round-and-round the circuit, and the only heat losses will be from tank, equipment and pipe sufaces. The system volume is small in comparison to that of a once-through RO plant, so the temperature of the system will rise until it is quite noticeably above ambient.

Substitute some figures for hydraulic oil as appropriate.

The-long-and-the-short of it is that one must expect a rise in temperature if the hydraulic system is running and does little work, and there is nothing to be done to prevent/correct it. All one is observing is the Laws of Thermodynamics in action!

(BTW: PWSlack is a username and has no gender)

adiabatic - Now that's a three dollar word!

So that explains why when I use my stair-master for about an hour, my sweat boils when it hits the hydraulic cylinders! I always thought it had something to do with the friction of viscous fluids passing through the cylinders!

when you do a mechanical work eg increasec pressure of hydraulic oil , not all energy goes to increase pressure , but some goes to heat it up aswell. Basically with the mrechanical work intermolecular positions gets changed and raises the temp. Same thing happens when you do cold extrusion.

Now solutions can be many

1. Give adequate fins to dissipate heat or increase surface area for more heat dissipation

2.water cool the oil in a heat exchanger

3 Ideally opt for Oil which is good for the temperature to which you are heating up now. The grade of oils are decided on the basis of temperature of use. So may be the hydraulic oil you already have is designed to run at higher temp. Please check that.

4 Keep the pump in operation intermittantly by storing high pressure oil and also have an accumulator . So pump will get time to cool off.

Seems to me that if it was running good at 35C and now is running 50C, then either the workload on the system has increased, or there is a malfunction in some part of the hydraulic system.

It would be helpful to know what the application is, along with a description of the hydraulic system. Are there coolers for the hydraulic oil? If so are they getting proper cooling flow?

Steve,

You're right. If it was once working fine within manuf. parameters and then is now changed when nothing has been done to the system, all of the calculation in the world, which is by the way OK, will not tell the guy the answer he needs.

The extra temperature is produced mainly by 'slippage' in the pump. The pump is worn past the correct tolerances. Slippage or leakage internally in rams will be indicated by other phenomena, usually.

I guess I should have addressed this to the guy himself but it's too late now. I just trust that he'll read it

Sorry, Man.

He needs to tell us more about the system. Is he running motors, rams?

Cheers,

Stu.

A very good example of a Highly Efficient Hydraulic Circuit that almost everyone in the Industrial World is familiar with is a Battery Operated Fork Lift that has Hydraulic Lift and Tilt.

This circuit never gets even warmed up since the only time the pump runs is when the Mast is raising or the Tilt is moving either direction.

The work accomplished can be very high but all the pump output, except for inefficiencies, is doing work at the rate required for the load at the moment. A circuit of this design will run all day without heating and without a Heat Exchanger of any kind.

BTW, If the operator holds the Tilt or Raise Handle after the Cylinders are at the end of their stroke, oil tempertture will steadily increase as the oil is forced through the Relief Valve at maximum system pressure.

The heating rate will be 2,544 BTU/Hour/HP of energy input.

How do some operators of Mobile Equipment get their Hydralics up to temperature in cold Weather? They run an acuator to the end of stroke and keep the valve shifted until the oil is hot. Then cycle all the actuators until the heated oil is distributed. That may require more Wasted Energy Input to heat up the cold returning oil several times.

The solution to Hot Running Hydraulic circuits UDAY KUMAR CHARAGONDLA is to design all hydraulic circuits to run without Wasted Energy.

That is often a challenge and if you are in sales you will never be low bidder. However, once your customers see the savings from operating low or no energy waste machines they will gladly pay extra up front. Been there and done that for several years.

Designing Low Energy Waste circuits is not Rocket Science it really boils down to common sense. Pay Me Now or Pay Me Later usually gets the attention of the end user. One big advantage is not hooking up a water line and having water mix with the oil when a Heat Exchanger fails.

It is easy to check any circuit for energy waste. In fact that is how your vendor determined what size Heat Exchanger to sell you. Why not look at the Heat Making devices, Low Efficiency usually Cheaper components, or circuit design areas that are inefficient, Then, design them out of the circuit to start with. It has always seemed logical to me after I found out where the HEAT was coming from.

Why not use an oil with a pour point appropriate for your ambient temperature ? The only reason to heat up an oil is that it is the wrong oil. For example, Lubrication Engineers,Inc. has a 5W20 non-syn with a pour point of -44* F. This oil is also perfect for hot summer days making it a year round oil and it will lower the temps you are used to seeing by foam control and greater lubricity.

Been doing that and recommending that for years. Especially for Mobile Equipment where there is a distinct possibility of adding Hydraulic Oil to the Engine CrankCase.

Seemed like a No Brainer to me after first hearing from a customer who was already doing it for several years.

I agree. I would also check the quality of your fluid. You may find your problem by moving a pressure gauge around comparable systems and making note of the bypassing problem or the leak bypass problem when you find them.

What you've told us is that the "machines" are at near 100% duty (compressing and adiabatically heating) for not less that 10 minutes straight. Was this always the case? If not, then the problem could be one of perception only. It would help to know more about those "machines"...to better make informed guesses you are seeking.

I skimmed the answers so far and I didn't see the cause I know has caused overheating on a hydraulic system I worked on.

With a variable volume 2000 psig pump,

-the normal minimum discharge back to reservoir on that pump was supposed to be about 1/10 gallon/minute.

-the actual leakage was about 10 gallons/minute.

The minimum flow bypass is the pump lube and heat dissapation and on that Racine pump was thru a 1/2" tube from the pump back to the reservoir.

The solution was a new pump as this was a new system under warranty.

CoronaCameraMan wrote:

"-the actual leakage was about 10 gallons/minute."

That leakage (Bypass) in the pump was all oil that had been pressureized but returned to tank without doing work. All the Work Energy wound up as HEAT.

I use an Infrared, Non-Contact, Hand Held device such as shown here:

http://www.me-us.com/temperature/

when a normally cool operatiing hydraulic circuit suddenly starts OVERHEATING.

These, Guns, I call them, work best when a system is cooled and then restarted. The offending component wil immediately show a noticeable temperature difference from its Inlet line to Outlet line. You merely point the Reader End of the Gun at the suspect component and read the temperature across it The one I use has a Laser Beam so you can point at something several feet away and no exactlly what area you are reading. Found a bypassing valve on a Rolling Mill circuit that was 30" in te air one night.

They can also be used on a running, overheated circuit also since the bypassing device will have a temperature increase across it. However, the temperature difference is not usually as noticeable. With experience you can still find worn or out of adjustment components though.

Try one, you will like it in those Trouble Shooting situations where Over heating is the problem.

I also would like to see any new circuit, that was running correctly and up to normal operating temperature, have a Temperature Reading at the Inlet and outlet of of all components recorded on the schematic. That would make it easy to spot a problem component later on when the circuit is showing a heat related problem.

Is your heat exchanger working well? (dirty or filthy water pipes)

Is the temperature on your water for cooling ideal?

Is the flow correct?

Have you changed any pressure reducing valve or placed more pressure on operation?

I live in the Pacific Northwest USA and only experience with hydraulics is with 3 of my bulldozers. No problem with overheating, the weather is cooler.

A missionary friend in Alaska did have a overheating problem with his 1992 4wd rig. At -50 degrees F the engine temperature was + 230 Degrees F. At 1,500 miles or 3 months the transmission fluid started to get black and burn and the transmission started to slip and become unreliable. He had a 100,000 warrenty but the dealer in Canada was 100+ miles away.

He had to go to Pennsylvania for medical reasons. I had talked to the Amsoil distributer and he suggested changing the automatic transmission fluid to of course Amsoil.

His data was that petroleum based transmission fluid started to burn at 285 degress F and left a carbon deposit in the system. This deposit and lack of fluid caused major problems and failures.

Amsoil transmission fluid starts to evaporate at 600 degress F and leaves no deposit.

My friend had tried all of the ideas from the regular auto industry but none had worked.

He stopped in Olympia (1,500 miles from where he started and the transmission was acting up and the smell of the fluid was gut wrecnhing when we took it out) and we changed all of the fluids in the 4wd rig to Amsoil.

He left the next morning. I got a call at about 10:00PM from Spokane, WA. (350 miles away) "Brother I don't have a transmission problem anymore. The temperature gage for the engine is running at about 190 degrees F. That is the lowest since I bought the rig."

He continued to have the rig for years after that and didn't have anymore overheating problems.

Long story. Now is there anyway that this hydraulic oil could be overheating because that it needs to be changed. Are there any manufacturers of synthetic hydraulic oil.

Hope this helps. Keep informed of results.

"Now is there anyway that this hydraulic oil could be overheating because that it needs to be changed. Are there any manufacturers of synthetic hydraulic oil."

Overheating will deteriorate the oil and deplete the additives that make it suitable for a given application.

I have advised changing oil in many hydraulic circuits that had the same burned smell and was very thin even when cold.

If the circuit continued to run hot and a Heat Exchanger was not an option then I recommended 10W40 or 20W50 engine oil and had great success in rejuvinating sluggish and low powered systems.

Most of this was done before the days of Synthetic Oils or I would have tried that type oil also.

I also had transmission problems on several General Motors cars after 70-80,000 miles of road travel in my sales days. After doing the transmission oil change on a 71 Chevelle and finding how much it improved the shifting I religiously started changing transmission oil every 10,000 miles. On the forst change I braze a 1/2-20 Nut on the Transmission Pan and plugged it with a bolt and a fiber washer. From then on it was merely a drain/refill situatuation at every other oil change.

Drove several more cars well over 100,000 miles without a transmission problem except for the later Front Wheel Drive jobs with everything so compact and lightly built. On those cars I went to 5-Speed transmissions and got back to the old days shifting gears.

Please change the hydraulic oil in the tank and clean the oil tank including pipings this would solve your problem

we are almost reaching the summery. The machine may have different grade fluid than what was observed in other machines. Check aand Change if required.Also clean the resorvoir and filter the oil..Secondly there could be fault in the relief valve, or leakages in Hydrulic pumps which can result into overheating , if the oil found OK. I think all these checkings be done and results be seen/compared with other similar machines. If a Pressure gauge is there , it is a best guide. Also need to check if the temperature is remaining steady at Eleevated level or it is continuously going up?

Is there either an air cooled radiator or a water cooled heat exchanger on the hydraulic units. In my limited hydraulic experience, I have never seen a system without coolers and/or heaters to maintain an optimum range for oil operating temperature. It is not unusual to have a radiator or heat exchanger dirty, or a control valve causing temperature control problems.

I saw where you are monitoring fluid temp; and I saw where some have discussed handheld heat sensor. Have you determined where the hottest temp readings can be measured? This direct approach--where there's most heat there's most burning--might lead you to the defect.

Any bypassing at any control device will cause heat. The bypassing is usually due to wear, the heat and any unfiltered wear metal will cause the wear and heat to rise further. Another problem can be inadequate design which barely removes heat when new and soon fails to keep up, then the above wear cycle starts. More heat less viscosity, less film strength and greater bypass.

REGARDLESS OF WHERE IN THE SYSTEM IT OCCURS, THE ANSWER ALWAYS INVOLVES FRICTION. Whether it's passing through pipes, valves, filters, cylinders,pumps or simply being pressured, it all comes down to friction. Even at the atomic level the atoms are moving against each other, and where two or more particulates rub each other, FRICTION (like crap) occurs!

tmf

1) The internal seals must have worn out

2) check for oil ( its corectness )

3) is there any air trapped

the hydraulic system heated up... due to

1) low oil level in the reservoir.

2) wearing of internal valve surface of control valves, pumps, actuators. leads to internal leakage followed by pressure drop. finally heat.

3) poor efficiency of the oil cooler.....

Give us some parameters, please.

I HAVE A WIRE CRIMPING MACHINE AND IT BEEN RUNNING MANY YEARS ,,,NOW OIL GETS VERY HOT JUST WHEN SITTING IDLE AND PUMP IS RUNNING NO GEARS EVEN TURNING I THINK ONE GUY N HERE MENTIONED PUMP FAILURE OR WORN OUT ,,IT SWIMS IN OIL BUT I NOW DRIANED NEW OIL OUT AS IT DID NOT HELP N DONT WANT TO BURN THE OIL LET MEKNOW ANY IDEAS U MAY HAVE TO HELP ME ITS BEEN A LONG TIME IVE BEEN TRYING THIS ,,,CLEVELAND OHIO ,,, READY TO CALL SOMEONE FOR BACK UP HELP!!!!

Dear Mr.CHARAGONDLA,

Why oil (Fluid or Water) getting heated.

Pl. recollect the studies you (we ) made during college days.

In the Pumps chapter, the output power from the motor is input power to the pump. There will be a small power loss due to friction in the bearings, fricton around the impeller due to fluid contact, by way of churning etc., etc. the balance is taken water H.P or shaft H.P. The difference of pump input power and shaft power is converted as heat, and transferred to oil or water.

The heat liberated will be at the rate of 2545 BTU/HP., or 860 KCal./KW and this heat will be absorbed by the circulating fluid (oil or water) and temp. rise will be depending upon the flow rate, sp. heat, of fluid, initial temp. etc.

This heat in the oil to be removed through oil cooler - through mere cooling coil or forced lubrication - which will be decided by thermal rating.

DHAYANANDHAN.S