Hydraulic cylinders are used in many applications and you can rarely miss out to have one for your application. Its applications cover different works and industrial purposes including hydraulic presses, packing machines, cranes, and forges; and also a wide range of mobile applications such as construction engineering, agricultural machines, and marine equipment. Without them, you cannot think of the operations like excavators, balers, loaders, man-lifts, drill-rigs, and a similar list of a whole bunch of series including the operations of equipment like booms, lifts, arms, buckets, etc. Whenever any application, regardless of the industry it belongs to, demands operations like pushing/pulling or lifting/lowering, the hydraulic cylinders come out as the only effective solution.
Selecting the right cylinder for an application is critical to attaining maximum performance and reliability. That means you need to take several parameters into consideration.
This article is intended to guide you about all the aspects of a hydraulic cylinder and help you know its construction and mechanism better.
Table of Content
- 1 What is a Hydraulic Cylinder and Why do you Need it?
- 2 The Mathematics of Hydraulic Cylinders
- 3 Hydraulic Cylinder Types
- 4 Hydraulic Cylinders Based on Design Principles
- 5 Hydraulic Cylinder Spare Parts and Accessories
- 6 Pressure Intensification In A Hydraulic Cylinder
- 7 How To Test A Hydraulic Cylinder
- 8 Effective Hydraulic Cylinder Repair Tips
- 9 Disassembly and Inspection Of Hydraulic Cylinder Spare Parts
- 10 Hydraulic Cylinder Assembly
- 11 Conclusion
What is a Hydraulic Cylinder and Why do you Need it?
A hydraulic cylinder is a mechanical actuator that converts hydrostatic energy into mechanical energy and is capable of producing works like pushing, pulling, lifting, and lowering for different applications. Don’t get confused thinking that hydraulic cylinders are similar to the hydraulic motors. The basic difference between these two is that cylinders carry out a linear motion instead of the rotary motion carried out by the motors. If we are to give you an idea about the physics of a hydraulic cylinder we would state the basic relation of the force with the operating pressure and piston’s effective area. Here, the force F (measured in Newton) is directly related to the maximum operating pressure p (measured in Bar) and the piston’s effective area A (measured in square centimeters).
The Mathematics of Hydraulic Cylinders
The equation denoted by F = p · A [in kN] can help you calculate the actual force you can get using it.
Here, A is the effective area of a cylinder which is the difference between the area of the piston and the piston rod. From the equation, we can see that the force to be produced is proportional to the effective area of the cylinder at constant pressure. It means the larger the area is, the larger the force will be. These are also known as linear motors. The rotating motors, meaning the hydraulic motors, can also be denoted by a similar mathematical equation to calculate its torque based on its displacement and max operating pressure.
Hydraulic Cylinder Types
There are two types of hydraulic cylinders available based on functional properties such as the double-acting hydraulic cylinder and the single-acting hydraulic cylinder. They are how their names suggest, single-acting cylinders are the ones with a single effective area located on the bottom of the component that is able to exert force only in one direction. And the double-acting cylinders are the ones having the effective area on both sides of the piston that can exert the force on both directions. Depending on the demand of the application, the double or single acting hydraulic cylinder is used in different mobile and industrial hydraulic systems. The most popular design principles of a hydraulic differential cylinder are the tie rod, roundline, and mill type cylinder.
Hydraulic Cylinders Based on Design Principles
Tie Rod Cylinders
One of the most commonly used hydraulic cylinders are the tie rod cylinders. These are used in different kind of manufacturing devices, machine tools, plastic machinery, automotive industry, and transfer lines. The best feature for which it is so widely used is its compact design that is excellent for saving space. This is the reason why it’s incredibly suitable for medium to heavy-duty industrial applications.
The top and the bottom of the tie cylinder and also the cylinder barrel are joined by four tie rods. Elongated tie rods at the cylinder head or base can be used to mount the cylinder. This hydraulic cylinder has a wide variety of standard options including the standard mounting styles. The wide mounting range of the tie rod cylinder includes options like pivot and trunnion, and threaded holes and subplate mounting.
Another widely used hydraulic cylinder is the heavy-duty roundline cylinders and mill-type cylinders. Any industry and applications related to mechanical engineering need the use of such cylinders. They are widely used in the mobile machinery industry, steel and iron industry, civil engineering, ship-building industry, presses, cranes, and different offshore applications. The best feature about this type of hydraulic cylinder is their robustness. In fact, the mill-type hydraulic cylinders are considered the toughest one having a pressure rating up to 250 bar.
The top and bottom these cylinders and the barrel comes welded and screwed having been tightly joined with bolts or retaining rings. They are designed to provide constant high performance in several applications. Roundline cylinders have much lesser mounting possibilities compared to the tie rod cylinders, however, standard mounting styles like clevis, trunnion, flange, and foot mounting are still possible.
Hydraulic Cylinder Spare Parts and Accessories
Now time to discuss the hydraulic cylinder spare parts and accessories that come with the hydraulic cylinders. A hydraulic cylinder includes piston seals and complete gland cartridge kit with seals. And the hydraulic cylinder accessories include the brackets and the alignment couplers.
As we said, the spare parts of a hydraulic cylinder are mainly seals such as the piston seals or the gland chevron seals. If the operating conditions are normal, it is rare to see any replacement required for the piston rod of the hydraulic cylinder or the cylinder body.
When it comes to accessories for hydraulic cylinder, we see it includes different brackets and rod-end clevis, knuckles or the spherical bearing eyes for supporting various mounting styles.
It is essential to follow the cylinder manufacturer guidelines and specifications for using spare parts and accessories for hydraulic cylinders. To avoid misfits that cause leakage or potential damage to the cylinder bore or the piston rod, you always need to use the genuine parts. Another important thing to note is that replacing/repairing the seal kits can only be done if you are sure that the sealing compound, seal type, and seal quality are alike or comparatively better to the original one.
Pressure Intensification In A Hydraulic Cylinder
If you are wondering what is the best way you can test the integrity of the piston seal in the double-acting hydraulic cylinders, then there is an easy bench-test to help you do this. However, it includes the intensification of pressure in the hydraulic cylinder. This doesn’t necessarily mean to be unsafe unless you have a better understanding of what does this concept really mean, but if you don’t understand it well enough then you surely have reasons to worry regarding the safety. Here we’ll be discussing the threats of intensification in a double-acting hydraulic cylinder.
Force in a hydraulic cylinder is denoted by the product of pressure and area (F = p x A). In a double-acting cylinder, the effective area is not even on both sides, as a result, the force produced by the piston and rod sides of the hydraulic cylinder is unequal as well. It means that if the effective area of the rod side is half what it is for the piston side, it will produce the force accordingly meaning the force will be half of the piston side for a similar amount of pressure applied on both of them.
The transposed version of the equation F = p x A is p = F/A which means the pressure is equal to the force divided by the area. If the cylinder’s rod side is supposed to resist the force developed by the cylinder’s piston side, with only half of its area, then the amount of the pressure has to be doubled. For example, if the piston side is applied a pressure of 3,000 PSI, a 6,000 PSI pressure will be needed on the cylinder’s rod side to produce an equal amount of force. This is how pressure intensification occurs in double-acting cylinders. Important to take note that pressurizing a cylinder that is rated at 3,000 PSI, to 6,000 PSI, can bring some really devastating consequences.
In case, the pressure is applied at the piston side of the double acting hydraulic cylinder and simultaneously the fluid is restricted to escape from the rod side, the pressure will be intensified in the rod side until the forces are balanced or the hydraulic cylinder catastrophically fails.
One particular incident that one of our members experienced could be described somewhat like the following:
He said that his boss had tried to press out a pin at a day when the ambient temperature was minus 36 degrees and cylinder temperature was minus 10 degrees. He added that his boss had then turned on the pump and had moved the lever only to had found the gland of the cylinder that was 7.5’’ in size and had an operating pressure of 2,500PSI, had blown out.
From this narration, it should have easily understood that the cylinder’s gland blew out because of pressure intensification. This happened due to the blockage between the cylinder’s rod side and tank that resulted due to the cold conditions. Actually, the air temperature had fallen below the hydraulic oil’s port temperature, as a result, the oil couldn’t flow.
This should draw the line for the potential danger of pressure intensification in double-acting cylinders. And if you fail to consider its implications, it may prove to be a costly mistake.
How To Test A Hydraulic Cylinder
We’ve discussed the dangers of pressure intensification in a double-acting hydraulic cylinder. Now we’ll explain the use of intensification effect while testing the integrity of the piston seal for such cylinders. But before getting into this test procedure, it is strongly advised that the danger associated with the procedure is completely understood as described above.
The conventional testing carried out to check the integrity of a double-acting cylinder piston seal is to pressurize it at the end of the stroke to measure if any leakage gets past the seal. This is commonly known as the “end-of-stroke bypass test”.
However, the end-of-stroke bypass test has limitations, and the major one is that it doesn’t usually expose ballooning of the cylinder tube originated due to hoop stress. To test this, the ideal way would be to conduct a piston-seal bypass test at the mid-stroke. So the difficulty that comes along this test is that the force created by the cylinder is supposed to be mechanically resisted, but that is not really practical for high-pressure cylinders with a larger diameter. However, conducting a mid-stroke bypass test hydrostatically can be done by using the intensification effect.
The test procedure is as follows:
- Secure the hydraulic cylinder with the service ports up.
- Both sides of the cylinder should be filled with the hydraulic fluid through the service ports.
- Follow the right procedure to connect ball valves, gauges, relief valve, and directional control valve.
- With ball valves open, stroke the hydraulic cylinder with the directional control valve few times to extinguish all remaining air from the cylinder – but do not ‘diesel’ the cylinder.
- Correctly position the piston rod at mid-stroke and close the ball valve.
- Having made the adjustment on the relief valve to back out, the direct flow will be made to the cylinder’s rod side.
- Start increasing the setting of the relief valve until the rated pressure is displayed on the gauge.
- Close the ball valve and center the directional control valve. Keep in mind that the hydraulic power unit used in the test has a protection for over-pressure.
- Keep records of the respective pressure gauge readings and monitor the change over time.
For an effective area ratio of 2:1 between the piston and the rod side of the cylinder, a 3,000 PSI pressure on the rod side will give a reading of 1,500 PSI on the piston side. To avoid any problem with piston seal or tube, the differential pressure should be maintained across the piston.
Never, direct the flow to cylinder piston side with the ball valve closed. Else, it will end up in the failure of the cylinder and an injury could result as well. Always use the required personal protective equipment while carrying out this test or any other hydrostatic test.
Effective Hydraulic Cylinder Repair Tips
The cylinders are a very common hydraulic product group like the pumps and motors. They are also less complicated than most of the other hydraulic components. So, they are comparatively easy to repair. For this reason, many hydraulic cylinder owners carry out the effective repairs to hydraulic cylinders by themselves. However, to get the best results, there are certain hydraulic cylinder repair tips that you should follow.
We will try to provide a guide below to carry out effective repairs to cylinders. The repair work can be carried out in-house but it depends on the amount of wear or damage. And in case of taking it to a repair shop, you need to check how well-equipped the shop is to get your job done. For any repair, one of the most important things is the economics involved in repairing a cylinder. However, it depends on the cost and availability of a new cylinder as you do not want to spend a price for a repair work that is close to a new one.
Disassembly and Inspection Of Hydraulic Cylinder Spare Parts
Usually, a cylinder is removed for repair only when an internal or external leakage is spotted. One of the most important hydraulic cylinder repair tips is to get the cylinder parts disassembled and have a close inspection on them, then they will expose problems that may remain unnoticed otherwise.
In case the piston is terribly eroded, missing, or distorted; the barrels will seem to be oversized and have bulged in service. In such a case, the complete cylinder or particularly the barrel needs to be replaced. Replacing the piston seal only and not the barrel will be considered as a short-term fix and further comprehensive maintenance will be required at the quickest possible time.
While the rod seal gets distorted, generally it is the guide bush that gets extremely worn or the rod which gets bent. For both these cases, the resulted weight of the rod causes failure on the seal. The replacement of the rod seal would be considered as a short-term fix unless the root cause of the problem is identified and rectified.
The rod needs to be inspected for cracks with the dye penetrant at every point where the cross-section seems to be changing. While inspecting the chrome surface, if one side comes out dull and other one is polished, then the rod is bent. Before a cylinder is taken for repair, the straightness of the rod should be checked. For that, the rod is placed on rollers and run-out is measured using a dial gauge. It is positioned to maintain the maximum distance between the rollers (L) and the run-out to be measured at the midpoint when the distance is L/2.
Mostly, bent rods are straightened in a press. Straightening the rods without causing any damage to the hard-chrome plating is still possible, however, if it gets damaged, the rod has to be either re-chromed or replaced.
In case, there is any pitting or scoring in the chrome surface of the rod, the efficiency and service life of the rod both will be decreased. For polishing the minor scratches in the chrome surface, a strip of fine emery paper can be used in a crosshatch action. But if it is badly pitted or scored, then the rod has to be either re-chromed or replaced. Machining a new one from the hard-chrome plated round bar is the best economical solution for a small-diameter rod.
For light-duty applications, the rod of the cylinder is supported directly on the material of the head. This material typically consists of aluminum alloy or cast iron. In between the rod and the head, there is a metallic/nonmetallic guide bush ( the wear band) fitted for the applications where the rod has to bear higher loads.
In the case of the rod taking direct support on the head, an internal micrometer or vernier caliper should be used to measure the internal diameter of the head. The measurement should be taken in two positions, 90 degrees apart for verifying the ovalness. The inside diameter of the cylinder head should be within the nominal rod diameter with an additional 0.004”. For instance, for a rod having the nominal diameter of 1.5”, the head’s inside diameter should not be exceeding 1.504”. Anything beyond this measurement will make the rod to load the seal of the rod which will cause a premature failure of the seal. So the head needs to be sleeved with a bronze bush otherwise be replaced with a new cylinder head.
The lands of the seal grooves in the head will not be affected very much with the minor scoring on the lands unless the maximum diameter across the lands remains within the nominal rod diameter and an additional 0.016”. Like the example above, a rod having 1.5” inside diameter of the head that is measured across the lands of the seal grooves will have to remain within the 1.516”. And again, the seal lands measuring beyond this tolerance will result in a reduced service life of the rod seal. In such case, the head needs to be replaced with a new one that is machined from a similar material.
The barrel needs an inspection for internal scoring or pitting because the efficiency and the lifespan of the piston seal will be affected in case any pit or score is spotted. So, the barrel has to be flawless by removing damage, otherwise replaced. Pitting or scoring that is less than 0.005” deep can be removed on small diameter barrels by the engine-cylinder honing tool. The barrel needs to be smoothed evenly across the full length.
The maximum diameter of the standard piston seals can be measured by calculating the nominal bore diameter with an additional 0.010”. For instance, the barrel with a 2.5” bore diameter can have a maximum bore diameter of 2.510” after honing and should not exceed it. This size could be checked at various points along the cylinder barrel by using an internal micrometer.
If pitting or scoring is found at more than 0.010”, the further honing of the barrel is needed to accommodate the oversize seals, otherwise replaced. The best economical solution for a small diameter cylinder is to manufacture a new barrel from honed tubing.
The barrels with a large diameter or inch-size could be salvaged by honing either 0.030” or 0.060” oversize and getting fitted to the respective oversize piston seals. However, the oversize seals for the metric-size cylinders are not so available. Therefore getting the metric-size cylinders salvaged by fitting oversize seals is not likely to happen always.
The pistons are one of the most vital hydraulic cylinder components used in light duty applications. They are generally made of aluminum alloy or cast iron. The pistons operate having direct contact with the hydraulic cylinder bore. A hydraulic cylinder’s function is related to the diameter of the piston and the diameter of the cylinder bore. The cylinder function is not really affected with minor scoring on the piston’s external diameter unless the minimum diameter of the piston is equal to or greater than the nominal bore diameter of 0.006”. It can be verified by an external micrometer. For instance, for a nominal barrel diameter or 2.5”, the minimum diameter of the piston would be 2.494”. If the measurement of the piston diameter is outside this range, it needs to be replaced with a new one machined from the similar material.
In the applications that have high-loads on the rod, the piston and barrel have a nonmetallic wear band fitted between them. If the cylinders are fitted with piston wear bands, these need to be replaced or repair.
Do not measure old seals while ordering seals from the supplier this is a common yet bad practice. Because there could be a previous incorrect seal installed or the seal could shrink or swell in service as well. To verify if the correct seals were supplied, measure the seal grooves using a vernier caliper and provide the measurement to the seal supplier.
Hydraulic Cylinder Assembly
When it comes to hydraulic cylinder assembly, at first, a complete and thorough cleaning of all the hydraulic cylinder parts is needed using a petroleum-based solvent and then getting them blow-dried using compressed air. During assembly, coat all the parts with clean hydraulic fluid. Also, make sure the seal grooves are clean and no nicks and burrs are involved before making any installation. To avoid damaging the seal, do not use a screwdriver or any other sharp object while installing seals. After getting the cylinder assembled, plug the service ports to restrict ingress of dirt or moisture.
If you are interested in applications where a hydraulic cylinder is a must-have component, you probably know the importance of this information about a hydraulic cylinder. We tried to cover all the important aspects of a hydraulic cylinder such as its types, mathematics, pressure intensification, spare parts and accessories, disassembly, inspection, assembly and most importantly hydraulic cylinder repair tips. We hope you have found this article helpful for your quest of achieving knowledge on hydraulic cylinders.
Feature Image Source: Hydraulic Cylinders Manufacturer Company