CAB's in-depth knowledge of ANSI rated material grades and equivalents helps provide our growing customer base with engineering alternatives that offer cost saving potential to their end customers including longer cylinder life and better wear characteristics. CAB has an extensive network of established ISO and IATF certified factories whose primary equipment and processes are geared to the manufacture of cylinder components. This means that CAB can match the facility's production strengths and knowledge to our customers' needs.
Welded body hydraulic cylinders dominate the mobile hydraulic equipment market such as construction equipment excavators , bulldozers, and road graders and material handling equipment forklift trucks, telehandlers, and lift-gates. They are also used by heavy industry in cranes, oil rigs, and large off-road vehicles for above-ground mining operations. The piston rod of a hydraulic cylinder operates both inside and outside the barrel, and consequently both in and out of the hydraulic fluid and surrounding atmosphere. Wear and corrosion resistant surfaces are desirable on the outer diameter of the piston rod.
These coatings can be finished to the desirable surface roughness Ra, Rz where the seals give optimum performance. All these coating methods have their specific advantages and disadvantages. It is for this reason that coating experts play a crucial role in selecting the optimum surface treatment procedure for protecting Hydraulic Cylinders. Cylinders are used in different operational conditions and that makes it a challenge to find the right coating solution.
In dredging there might be impact from stones or other parts, in salt water environments there are extreme corrosion attacks, in off-shore cylinders facing bending and impact in combination with salt water, and in the steel industry there are high temperatures involved, etc.
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There is no single coating solution which successfully combats all the specific operational wear conditions. Every technique has its own benefits and disadvantages. Piston rods are generally available in lengths which are cut to suit the application. As the common rods have a soft or mild steel core, their ends can be welded or machined for a screw thread.
The forces on the piston face and the piston head retainer vary depending on which piston head retention system is used. If a circlip or any non preloaded system is used, the force acting to separate the piston head and the cylinder shaft shoulder is the applied pressure multiplied by the area of the piston head.
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The piston head and shaft shoulder will separate and the load is fully reacted by the piston head retainer. If a preloaded system is used the force between the cylinder shaft and piston head is initially the piston head retainer preload value. Once pressure is applied this force will reduce. The piston head and cylinder shaft shoulder will remain in contact unless the applied pressure multiplied by piston head area exceeds the preload.
The maximum force the piston head retainer will see is the larger of the preload and the applied pressure multiplied by the full piston head area. The load on the piston head retainer is greater than the external load, which is due to the reduced shaft size passing through the piston head. Increasing this portion of shaft reduces the load on the retainer. Side loading is unequal pressure that is not centered on the cylinder rod. This off-center strain can lead to bending of the rod in extreme cases, but more commonly causes leaking due to warping the circular seals into an oval shape.
It can also damage and enlarge the bore hole around the rod and the inner cylinder wall around the piston head, if the rod is pressed hard enough sideways to fully compress and deform the seals to make metal-on-metal scraping contact. The strain of side loading can be directly reduced with the use of internal stop tubes which reduce the maximum extension length, leaving some distance between the piston and bore seal, and increasing leverage to resist warping of the seals.
Double pistons also spread out the forces of side loading while also reducing stroke length. Alternately, external sliding guides and hinges can support the load and reduce side loading forces applied directly on the cylinder.
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Hydraulic cylinders form the heart of many hydraulic systems. It is a common practice to dissemble and rebuild an entire device in the case of hydraulic cylinder repair. Inspection of the leakage issue and scrutinizing cylinder parts especially the seals is helpful in recognizing the exact problem and choosing the repair options accordingly.
Steps involved in the repair of hydraulic cylinders: . First of all, you should place the cylinder in a suitable location, which has sufficient space to work. If you are working in a cluttered space, it will be difficult for you to keep track of opened up parts. After bringing the cylinder to an appropriate spot, open the cylinder ports and drain out all the hydraulic fluid. The cover of cylinder can be removed by unscrewing the bolts.
Once you take off the cover, remove the piston by loosening the input valves. Once the piston is completely removed, you will be able to see multiple seals on different parts that are connected to the piston rod. First of all, you need to examine the piston rod to see if there is any damage.
If the shaft of the rod is bent or if the cylinder bore has scratches, then get them repaired at a professional repair shop. If the damage is permanent, then you can order or manufacture a new piston rod for your hydraulic cylinder. Piston seals can get damaged, be distorted, or worn. Such damaged seals can cause leakage of hydraulic fluid from the cylinder leading to lower overall pressure or inability to hold pressure. When such events occur, you know that these seals need to be replaced. The seals can be repacked with the help of a hydraulic cylinder seal kit.
These kits will have seals and suitable o-rings. Remember the size and type of old seal while removing them and fix the new ones accordingly. Make sure that you handle the new seals with utmost care so that they do not get damaged in any way. Before reassembling all the parts of your cylinder, you should clean and dry the cylinder barrel completely. Also clean the piston rod, shaft, and other parts of the cylinder.
Get the broken and damaged seals repacked. Then assemble the parts back on the piston rod. The assembly needs to be done in a reverse order. Once you have assembled all the parts back, put the rod into the soft-jaw vise and screw back the bolts onto the piston rod. If the parts of the hydraulic cylinder are severely damaged, then it is advisable to replace them with new parts with the help of a professional repair expert. By following the above steps, you can accomplish the task of hydraulic cylinder repair. Make sure that you prevent ingress of moisture or dirt after assembly of the parts is done.
Mounting methods also play an important role in cylinder performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer and avoiding wear. Common types of mounting include:. Flange mounts —Very strong and rigid, but have little tolerance for misalignment. Experts recommend cap end mounts for thrust loads and rod end mounts where major loading puts the piston rod in tension. Three types are head rectangular flange, head square flange or rectangular head. Flange mounts function optimally when the mounting face attaches to a machine support member.
Double rod-end cylinders are useful for moving two loads simultaneously, and they also eliminate the differential area between the rod side and blank side of the piston. With equal areas and cylinder volumes on both sides of the piston, a given flow produces the same extension and retraction speeds. Telescoping cylinders have two or more stages that, when fully extended, can produce a stroke that exceeds the length of the cylinder when fully retracted. Shown here is a cutaway of a six-stage single-acting model.
Most telescoping cylinders , Figure 6, are single acting, although double-acting versions are available. Telescoping cylinders contain five or more sets of tubing, or stages, that nest inside one another. Each stage is equipped with seals and bearing surfaces to act as both a cylinder barrel and piston rod. Available for extensions exceeding 15 ft, most are used on mobile applications where available mounting space is limited.
The collapsed length of a telescoping cylinder can be as little as 15 its extended length, but the cost is several times that of a standard cylinder that can produce equivalent force. Models are available in which all stages extend simultaneously or where the largest stage extends first, followed by each successively smaller stage. Short-stroke cylinders have a piston diameter that exceeds rod length.
They are used where axial space is limited and high force must be generated from a relatively low supply pressure. Ram cylinders are a special type of single-acting cylinders that have a rod OD the same diameter as the piston. Used mostly for jacking purposes, ram cylinders must be single acting because there is no internal cylinder volume to pressurize for retracting the rod. Ram cylinders sometimes are called plunger cylinders and are most often used for short-stroke applications. Most do not use return srpings, but, rahter, gravity or the load to retract the piston rod.
Short-stroke cylinders , Figure 7, generally have a rod length that is less than the piston diameter. It is used where high force must be generated from a relatively low supply pressure. Short-stroke cylinders also fit into a narrow axial space but require substantial radial width.
These cylinders lend themselves to air-operated, automation machinery. Tandem cylinders use multiple pistons connected through a common rod to generate relatively high force from a low supply pressure and small bore. Tandem cylinders , on the other hand, are designed for applications where high force must be generated within a narrow radial space where substantial axial length is available.
A tandem cylinder, Figure 8, functions as two single rod-end cylinders connected in line with each piston inter-connected to a common rod as well as a second rod which extends through the rod-end cap. Each piston chamber is double acting to produce much higher forces without an increase in fluid pressure or bore diameter.
Duplex cylinders also have two or more cylinders connected in line, but the pistons of a duplex cylinder, Figure 9, are not physically connected; the rod of one cylinder protrudes into the non-rod end of the second, and so forth. A duplex cylinder may consist of more than two in-line cylinders and the stroke lengths of the individual cylinders may vary. This makes them useful for achieving a number of different fixed stroke lengths, depending on which individual pistons are actuated.
Duplex cylinders have multiple pistons that are not connected to a common rod.
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Actuating individual piston chambers achieves multiple strokes. Diaphragm cylinders , Figure 10, are either of the rolling diaphragm or the short-stroke type. Both use elastomeric diaphragms to seal the barrel-piston interface. The short-stroke type uses an elastomer sheet secured between halves of the cylinder body and is commonly used for truck and bus air-brake applications. The rolling diaphragm cylinder has a hat-shaped diaphragm that rolls into the cylinder barrel as the piston advances.
Both types require very low breakout forces, have zero leakage, and are single-acting, spring returned. By eliminating the conventional sliding piston seal - and its inherent tendency toward stick-slip operation - diaphragm cylinders produce nearly frictionless motion, except for the miniscule amount of friction at the rod seal. The diaphragm's positive sealing also eliminates the potential for leakage around the piston. Cylinders - and all components for that matter - should be readily accessible to ease installation and subsequent maintenance.
If a fitting cannot be checked for tightness without first removing adjacent lines, for example, there is little incentive to bother fixing minor leaks that may occur. Consider all components and fluid conductors of the system to be elastic: they will flex and change length due to changes in fluid pressure, temperature, and strain. These changes are not minor.
A pressure pulse to 6, psi will elongate a steel cylinder with a in stroke by 0. If made of aluminum or cast iron, the cylinder can elongate about 2 to 2. If this elongation has not been accounted for in the design of the machine, the system eventually will leak, even if the latest fitting technology has been used.
If previous installations have continually leaked, take this as clear evidence that a new design approach would be beneficial. An industrial cylinder should have a design factor of about based on yield at rated system pressure. Many manufacturers of heavy-duty cylinders for mobile equipment specify a design factor. A 15,psi stress at rated system pressure, with smooth system operation and no pressure pulses, is considered conservative. System pressure spikes that cause 30,psi stress often are not alarming, but at 30,psi unit stress, steel's dimensional change is 0.
For a in. Dimensional changes in stressed cylinders, or those subjected to wide temperature changes, may further limit allowable working pressures. Large dimensional changes can seriously affect performance and life expectancy of nonmetallic cylinder seals. For example, extrusive failures of 80 Shore A durometer, synthetic Nitrile seals can occur when clearance exceeds 0. Such pressures can easily be reached in systems using differential cylinders or those with meter-out flow controls.
You must consider system shock pressures. If the hydraulic system contains speed control or energy-absorbing devices, pressure spikes can occur that are two to three times above normal system pressure. Therefore, determine the loading the cylinder will experience and then mount accordingly to maintain port seal integrity. Cap flange mounts are the same as c and d but bolt to cap not shown. Rectangular caps also are available. Trouble-free use of fluid power cylinders and their ability to serve and remain leak-free depends, in large part, on properly mounting the component for the particular application.
The designer must determine the loading the cylinder will experience and mount it accordingly.
The National Fluid Power Association, NFPA, has standardized on a number of dimensions for square-headed tie rod cylinders to promote cylinder interchangeability between manufacturers. Part of this standardization program includes cylinder mounting styles, which generally provide:. Straight line - Cylinders with fixed mounts that absorb the force on the centerline of the cylinder are considered the best for straight line force transfer.
Tie rods extended, flange, or centerline lug mounts are symmetrical and allow the thrust or tension forces of the piston rod to be distributed uniformly about the cylinder centerline, Figure Mounting bolts are subjected to simple tension or shear without compound forces; when properly installed, cylinder bearing sideloading is minimized.
Cylinder tie rods are designed to withstand maximum rated internal pressure, and can be extended at either end and used to mount the cylinder. When the tie rods extend at both ends of the cylinder, one end can be used for cylinder mounting and the opposite end can support the cylinder or be attached to the machine members. Flange mounts also are extremely good for straight line force transfer applications. Three styles available are head rectangular flange, head square flange, and a larger and thicker rectangular head with its own mounting holes; the same three versions are available for the cap.
Selection of a flange mount depends partly on whether the major forces applied to the load result in compression or tension on the piston rod. As with other NFPA standardized mountings, centerline lug mounts provide striaght-line transfer of force. Cap mounts are recommended for thrust loads while head mounts should be used where major loading puts the piston rod in tension.
Centerline lug mounts, Figure 12, absorb forces on the centerline; they are the least popular fixed mounting style. When used at higher pressures or under shock conditions, the lugs should be dowel pinned to the machine. Side mounted cylinders include side lug a , side end angle b , side and lug c , and side tapped not shown. These mounts produce a turning moment as the cylinder applies force to the load. Straight line, force not absorbed - Side mounted cylinders do not absorb force along their centerlines. These mounting styles have lugs on the end closures and one style has side-tapped holes for flush mounting, Figure The plane of their mounting surface is not through the centerline of the cylinder; for this reason, side mounted cylinders produce a turning moment as the cylinder moves the load.