It’s that time of year again and all over the country there are hydraulic systems overheating. Some of them overheat every summer, but most of them usually just run a little warmer with increases in ambient temperature. What is causing it? Often a dirty or poorly placed heat exchanger can be the culprit. Sometimes a bypassing check valve across the heat exchanger can make the system overheat. Unless the design of the system has been changed in some way, it is very unlikely that adding a heat exchanger is the answer. Systems that overheat are simply not operating efficiently and there is usually an easily corrected cause. Here are four of the more common causes of overheating that we find.
By far the most common cause of overheating that we find is an improper adjustment. On machines that usually do not overheat but are overheating now, this is the first thing we look for. The most common component that gets out of adjustment is the pump compensator. It is imperative that the compensator be set below the system relief valve. But for whatever reason, someone decides that the system would run better at a higher pressure. The compensator setting then gets increased, but the relief valve setting does not. Once the compensator setting approaches the relief valve setting, the relief valve starts to crack open and dump oil flow to tank. The more the compensator setting is increased, the more the relief valve opens and the more flow returns to tank. Instead of varying its flow to meet the demands of the system as it is designed to do, the pressure compensating pump moves to full stroke behaving as a fixed displacement pump. Any flow not used to move a load returns to tank through the relief valve. At system idle, ALL of the pump flow returns to tank. Since the resulting pressure drop doesn’t do any work, virtually all of the energy going into the system is converted to heat causing the fluid temperature to soar.
Changing the system pressure is usually a bad idea. Most systems have designer recommendations for system pressure and a lot of design criteria are taken into consideration to determine the optimum system pressure. Often the pressure is increased in an attempt to speed up the machine but this is a very inefficient way to accomplish this. Flow rate, not system pressure, determines the speed of the actuator. Yes, turning up the pressure will often also increase the flow, but the more efficient way is to open a flow control or raise the setting of the manual volume adjustment. In the absence of designer recommendations, we usually recommend that the system relief valve be set approximately 250 PSI above the pump compensator.
In Figure 1 above, the system is adjusted correctly. The pressure compensating pump is capable of delivering as much as 30 GPM. With only the directional valve on the left open, the system uses 10 GPM so the pump will stroke only enough to maintain 1200 PSI in the system delivering 10 GPM. When the second directional valve opens in Figure 2, the pump stroke increases to 20 GPM to maintain the 1200 PSI setting. But in Figure 3, the compensator setting has been increased to 1600 PSI. The relief valve however is set below that amount at 1450 PSI. In an attempt to reach 1600 PSI in the system, the pump will stroke to its maximum 30 GPM. The system only uses 10 GPM leaving 20 GPM to dump across the relief valve generating heat. We can calculate the heat that is generated using the formula HP = PSI X GPM X .000583 and we see that an extra 17 HP is generated. In Figure 4, the system is at idle. The pump will remain at full stroke however dumping its full 30 GPM across the relief valve. This generates a whopping 25 HP in excess heat!
Undersized System Piping
This is one that we find most often in systems that have been modified from their original design. One of the more common system upgrades is a higher flow pump to increase speed, but the system piping and hoses may not get the same upgrade. The result is that it takes more pressure just to push the oil to the actuators to do the work. For example, 20 GPM will flow through a #10 SAE hydraulic hose at a fluid velocity of 20 feet per second, but if the hose is replaced by a #12 SAE, the fluid velocity drops to only 15 feet per second. This simple size increase reduces the restriction by 25%. Tight radius bends in pipe will also increase turbulence in the lines. Whenever the system flow rate is increased by installing a higher flow pump, check pipe and hose charts to ensure the system can withstand the greater flow rate without an excessive increase in fluid velocity. In the absence of designer recommendations, we usually recommend that fluid velocity be kept between 2 – 5 fps in pump suction lines, 10 – 15 fps in return lines and 15 – 20 fps in pressure lines (for systems up to about 3000 PSI) Above 3000 PSI, the system designer will usually specify how many bends can be in the system piping and what radius they must be. This avoids the necessity of exceptionally large piping in higher pressure systems.
As components wear, internal bypassing increases. Oil that bypasses across the tight tolerances of a component undergoes an immediate pressure drop that performs no work. Heat is the inevitable result. The more bypassing that occurs, the more heat is generated. Some components are notorious heat generators even when brand new. Servo valves, proportional valves and flow controls all generate heat from the time they are new because they always have a pressure drop across them. So how do we know if a component needs to be replaced in order to keep the temperature down? The best way is to routinely measure the temperature gain at various components in the system. Keep a record of these measurements and use them to help locate troublesome worn components.
One of the primary purposes of the reservoir is to radiate heat to atmosphere. You may have wondered why your hydraulic reservoir is rectangular in shape while most other tanks you see are round. The most efficient use of volume, i.e. the maximum volume with minimum surface area, is a sphere. This may be desirable in some applications, but in a hydraulic system we want more surface area to radiate heat, not less. When a system is designed, the reservoir is sized based on a number of factors such as how much the oil level will rise and lower with the operation of cylinders, how long oil should remain in the reservoir to allow contaminants to sink to the bottom and how much heat will need to radiate to atmosphere. If a higher flow pump is installed, the oil will not stay in the reservoir long enough to dissipate heat. Also, when a variable displacement pump is used, oil from the pump case drain is ported directly to tank. A higher flow pump will also have a higher case drain flow. Most variable displacement pumps will bypass approximately 1 – 3% of its maximum flow rate. Case flow is very hot because it is oil that has just bypassed across the tight internal tolerances inside the pump. A higher case flow will raise operating temperature if a larger reservoir is also not part of the upgrade.