Over the years, I have made and seen some bad hydraulic system designs. For the most part, these have be easily fixed. Others we just suffered through the consequences of bad design. Let’s take a look at the 10 most common design blunders, identify the cause and prevent them from happening again.
Most hydraulic system issues are caused by: using the wrong oil, improperly sized suction lines and hoses, dirty oil with poor filtration, oil that is too hot or too cold, improperly sized reservoirs, undersized flow and pressure and a lack of maintenance.
Using the Wrong Oil
Without a doubt, oil is the most important component of any hydraulic system. Choosing the right oil can mean the difference between long life and constant problems. Oil provides two main functions: it is the method by which work is done and it lubricates the components.
The main factor is the oil viscosity. The viscosity pretty much sets the range of acceptable temperatures for you system to work at. Low viscosity, like ISO 22, will perform much better with cold weather startup. However, the upper limit on this fluid is not very high. If you work in cold climates, your mobile equipment should be be using low viscosity oil.
Conversely, if you are in the desert, cold temperatures may not be an issue, so we need better protection at high temperatures. Here an ISO 46 or 68 oil will be a better fit for your application.
To further complicate things, using a very thin oil will lead to power loss. A thin oil will leak past the components of pumps and motors. This loss of power creates even more heat making the oil thinner.
If your machine is inside and somewhat isolated from drastic changes in temperature, selecting an oil might be as simple as using and ISO 46 oil.
If you have doubts as to what oil you should use, contact your local hydraulics distributor for more information. In addition to viscosity information, they can instruct if there are any additives that may be beneficial.
Improper Suction Lines
Hydraulic components are not meant to run dry! They will destroy themselves within a few turns of the shaft if run without oil. A common mistake is to believe that a pump is self-priming or self-lubricating.
Starving the pump of oil can cause component deterioration or cavitation can occur. Cavitation is where the fluid won’t flow correctly and the pump will try to pull the oil in too fast. Small pockets of oil will instantly boil and then cool rapidly from the surrounding fluid. This is like setting of may small explosions in the system.
Avoiding this all starts with the location of the pump relative to the reservoir. Whenever possible, the pump should be at the same height as the reservoir or lower. Gravity will help push the oil to the pump.
Design your system for short suction line hoses or oversize them if you cannot. A good rule of thumb is that a suction hose should have a flow rate of 5 ft/s (1.5 m/s) or less. If you have a long run, you may want to pick the next larger size.
We also tend to put more component between the reservoir and pump than needed. Commonly, I see suction strainers, shutoff valves and filters. Once I saw a 5 psi check valve (The client couldn’t explain why either).
My personal preference is to filter on the return line (more on that in the next section) so any filtration here. If you must filter here, be sure that the filter has a low pressure bypass on it. This should be around 3-6 psi (0.2 – 0.4 bar).
Definitely keep the suction strainer in the reservoir to keep the large chunks from reaching the pump. It should also be at least 2″ (50mm) off the bottom so that dirt has a harder chance of getting sucked in.
Putting a shutoff valve is often a necessary evil. Starting the pump with it closed is a surefire way to kill you pump; however, it is handy when you need to replace the pump and don’t want to make a huge mess. I recommend using safety wire or zip ties to secure the valve in the open position. This will prevent most problems.
When you start your car, you want oil already available to lubricate the engine. We want the same for our hydraulic system; the oil already needs to be at the inlet of the pump. This can be done by loosening the inlet fitting after the reservoir is filled. Any trapped air in the line will escape, filling the inlet with oil.
If your pump has a case drain, make sure it is located on the top. Remove the hose and pour a little oil in. Replace the hose.
There are three locations to put a filter. On the suction line, right after the pump and on the return line.
Common logic is that putting the filter on the suction line would be the best. However, this usually isn’t the best way to filter. The first problem is that you can easily starve the pump if your filter gets clogged. A low pressure bypass valve will prevent starvation, but now you will be contaminating all parts of the system. In order for this to work, you will need a frequent PM schedule and probably be throwing away mostly clean filters.
The next logical thing is to put a high pressure filter right after the pump. The pump is most likely to be damaged by contaminates and we have lost the ability to filter them. On top of that, the filters need to be more robust to deal with the pressure and therefore, more expensive.
The problem with suction and high pressure filters is that they do not address the source of contamination; the cylinders. Yup, every time the cylinders stroke, dirt sticks to the oil rod surface. When it retracts, the larger chunks get wiped off. But the small ones, the ones that damage pumps, get ingested into the system and eventually the reservoir.
This is why having a return filter makes the most sense. Having a filter in the other two locations eventually makes the reservoir like the Great Salt Lake. Rivers flow into it, but nothing flows out. It collects everything.
No! We want to keep all components of our system clean. Having the filter on the return line will filter out the new contaminates from the cylinder instantly.
Return filters also have an advantage of a longer lasting filter, because the bypass setting can be higher.
Oil Too Hot
Overheating oil can be a big problem. It deteriorates seals, the oil itself and the lubrication that the oil gives. If your oil is cooked, you will know it by the awful smell. If you are using a biodegradable oil, it may smell like French Fries.
Be sure to look at the manufacturers recommended operating temperatures. Keep in mind that at sustained lower temperatures, the oil may eventually burn.
The trick here is to have a temperature switch on the reservoir that shuts off the system and add an oil cooler if necessary.
You also want to analyze your system to see where the heat is coming from. The main culprits are counterbalance valves, flow dividers and flow control valves. Try to minimize the effects or eliminate them.
Oil Too Cold
After some testing by a partner in Canada, we got a returned pump with a broken shaft. It didn’t take long for us to figure out what happened. Though our oil was rated for colder weather, the oil was very cold and the operator didn’t warm it up before using.
When working with cold oil, you need to allow the pump to circulate the oil at a low flow rate. This can take a while depending on reservoir size and flow rate.
This is practical in milder climates, but unacceptable in cold climates because it takes a while to get the oil warm enough. In this case, you need to add an oil warming option which is usually electrical heat. I’ve seen some systems where a propane system was used to heat antifreeze (many feet away) and a heat exchanger would transfer that heat to the hydraulic oil.
There were two causes to the pump failure. The first is that the operator was running the truck at high idle. The second was that the main relief valve was around 15 ft (4.6m) from the pump outlet. The relief valve opened at the right pressure, but because of the increased viscosity of the oil, the pressure at the pump was much higher.
The two combined caused the pump to be overloaded and the shaft snapped. For this reason, I recommend that a relief valve be mounted as close to the pump as possible.
Improper Sized Hydraulic Tank
For one of my first clients, I designed a machine that had 2 large ram cylinders. I had purchased an 80 gallon reservoir and was filling it when I realized, I don’t think the reservoir is big enough. I didn’t understand yet exactly how small it was.
I had sized the capacity with the 3:1 rule of thumb, where the reservoir capacity is at least 3 times the flow rate. But what I did not consider is how how much fluid those cylinders would use; 44 gal. My tank would be more than half empty when the cylinders were extended! The float switch would be triggered on every cycle.
A simple calculation ahead of time would have saved a lot of heartache and hassle. We ended up designing a new tank, that was much wider and the same height. Now, when the cylinders were stroked, the level in the tank didn’t change as much.
3:1 Rule of Thumb
Rules of thumb are well….rules of thumb. This one is no different. I’ve designed systems as low as 2:1 and know power steering systems in cars may be as low as 1:6. Is there anything wrong with this?
The 3:1 rule arose because it gave the oil time to cool before going out for another cycle. This issue can be mitigated by improving the efficiency of your system and adding an oil cooler. The other thing to watch for is running low on fluid. If you are going to go less than 3:1, be sure to install a float switch.
General Shape Guidelines
As mentioned above, my ram cylinders took up too much oil and significantly dropped the level of the tank. Since it was a submerged pump, all of the components were mounted on the pumps lid, making it difficult to change the distance the pump was submerged. In this case, it didn’t make sense to relocate all the components to use a taller tank.
Instead, we used a much longer and wider tank to solve our problem.
However, this isn’t a good idea in mobile applications. I once evaluated a tracked crawler machine that had a very long, but short reservoir. When testing the performance on steep slopes, we noticed that oil was coming out of the filler cap. Had we pointed the unit the other way, the suction port would have been too high (it was on one end) and we might have starved the pump.
For mobile applications where tilt is an issue, try to design a taller tank and put the filler cap and suction port in the center.
Small Hose Size
Often, small hose sizes are to blame for excess heat and poor performance. Often this is seen when cylinders are retracting and the return port sees more flow than the system flow. If the hose is too small, it will take more pressure to move the oil. This creates heat or impedes the motion of the cylinder.
The pressure it takes to move the oil increases with the length it must travel. If your hose run is long, you may want to increase the hose diameter just to keep the pressures down.
Before I acquired my log splitter, I was borrowing it from my neighbor. He had a single stage pump on it with a 6.5 hp gas engine. IT WAS SLOW. Painfully slow. If I were to put a larger displacement pump on it, it would be faster, but not have the required force. This would stall the engine.
However, there wasn’t a clear way to change this without increasing the size of the engine and pump.
Luckily, the pump went out on it and we got the opportunity to improve it. After some research, I found a two stage pump that would work with the gas engine. It provided high flow at low pressures and low flow at high pressures. This sped up the cylinder tremendously.
I usually get burned here by accident. The last time this happened was with an articulating lift device. I had designed the cylinder to be 94% efficient at the intended system pressure of 3000 psi (207 bar). We were using a piston pump to power the unit that was capable of a maximum pressure of 3000 psi. What I didn’t realize is that this is the standby pressure and not working pressure. The difference (margin pressure) was 250 psi (17 bar). Once friction in the cylinder and joints were factored in, I needed about 2800 – 2850 psi (193 -197 bar) to lift.
CRAP! When this function started to lift it was slow! Once the articulation got to about 15° though, the pressure had dropped to below 2750 psi and the cylinder would perform normally.
In this case, I should have designed to the working pressure of 2750 psi (190 bar).
We ended up solving this problem by increasing the cylinder bore by 1/4 inch (6mm).
Lack of Maintenance
Oil cleanliness is the leading cause of system failure. We’ve already discussed how to design for good filtration, but those filters need to be changed. Here’s where a good preventative maintenance program comes into play.
Oil right out of the barrel is not going to meet your cleanliness standards. If your system requires high levels of cleanliness, you may want to use an external filtration system before you fire up the pump.
Most cases, it is sufficient to let the pump idle and filter the oil naturally. This is easy to do with an open center system. On a closed center system you may need to manually open a path to tank. You’re going to catch the most dirt in the system in the first few hours or days of use. I recommend that you swap out the filter after a week. In a previous job, we made mobile trucks and would change out the filter right before we sent it to the customer.
After that, get this on a preventative maintenance schedule. This is going to depend on your duty cycle and operating environment. If this is unpredictable, take oil samples once a month. When you notice that the filter is clogged, in bypass mode or the oil is too dirty it is time to change the filter.
There’s a tension that exists between not ever changing the filter and changing it too often. I would lean toward changing a mostly clean filter rather than having issues with component failure. The small cost of a filter usually outweighs troubleshooting a system and the downtime that comes with it.
Finally, when you do need to service a component or fill the tank. Clean! We don’t want to introduce dirt into the system during service. So take a few minutes to wipe down all the surfaces with some industrial cleaner before breaking a line. Be sure to look for sources of debris that may fall from above like overhead cranes.
Feature image by Simon Speed, CC0, via Wikimedia Commons