Engineering Essentials: Cylinders

fig. 14. pivot mounts absorb force along centerline and actuate loads that travel through arc. cap trunnion (a), intermediate fixed trunnion (b) can locate anywhere between head and cap, and head trunnion (c) are versions of this style; only one of these versions is used at one time. the cylinders shown in figures 3 and 6 employ clevis mounting, which is a type of pivot mounting for loads that travel through arc.

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Pivot force transfer - Cylinders with pivot mounts that absorb force along the centerline should be used when the actuated load travels through an arc. There are two ways to mount a cylinder so it will pivot during the work cycle: clevis or trunnion mounts, Figure 14. Pivot mount cylinders are available with cap fixed clevis; cap detachable clevis; cap spherical bearing; and head, cap, and intermediate fixed trunnion. Special trunnion assemblies that provide gimballing action are available.

Pivot mount cylinders can be used in tension or thrust applications at full rated pressure, except that long stroke cylinders in thrust applications are limited by piston rod column strength. Clevis or single-ear mounts usually are an integral part of the cylinder cap although detachable styles are available and provide a single pivot for mounting the cylinder. A pivot pin of appropriate length and diameter to withstand the maximum shear load at rated cylinder operating pressure is included as part of the clevis mount. The fixed clevis mount is the most popular and is used where the piston rod travels a fixed arc in one plane. It can be used vertically or horizontally.

On long-stroke thrust applications, it may be necessary to use a larger diameter piston rod to prevent buckling or use a stop tube to minimize cylinder side loading in its extended position. Fixed clevis mounted cylinders do not function well if the path of rod travel is in more than one plane. Such an application results in misalignment and causes unnecessary side loading on the bearing and piston. For applications where the piston rod will travel a path not more than 3° either side of the true plane of motion, a cap spherical bearing mount should be used as well as a spherical bearing rod eye. Cap detachable clevis mounts are most often used for air or medium-duty hydraulic service.

Trunnion pivot mounts also are used when the piston rod travels an arc in one plane. Trunnion pins are designed for shear loads only and should not be used with bending stresses. The support bearings should be mounted as close as possible to the trunnion shoulder faces.

Head trunnion mounted cylinders usually can be specified with smaller diameter piston rods than cylinders with the pivot point at the cap or at an intermediate position. On head trunnion mounted long stroke cylinders, the designer should consider the over-hanging weight at the cap end of the cylinder. To keep trunnion bearing loads within limits, stroke lengths should be not more than five times bore size.

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An intermediate fixed trunnion mount is the best trunnion mount. It can be located to balance the weight of the cylinder or anywhere between the head and cap to suit the application. Its location must be specified at time of order because its location cannot be easily changed once manufactured.

Installation

Proper installation begins with machine layout; here are some rules:

• if high shock loads are anticipated, mount the cylinder to take full advantage of its elasticity, and don't forget: the fluid lines are along for the ride, hold fixed-mounted cylinders in place by keying or pinning at one end only
• use separate keys to take shear loads: at the head end if major shock loads are in thrust, at the cap end if they are in tension
• locating pins may be used instead of shear keys to help take shear loads and insure cylinder alignment. Avoid pinning across corners - this can cause severe warpage when a cylinder is subjected to operating temperature and pressure. Such warpage also is imposed on fluid connectors at cylinder ports, and
• pivoted mounts should have the same type of pivot as the cylinder body and the head end. Pivot axes should be parallel, never crossed.

Many fluid power cylinders incorporate cushions to absorb the energy of moving masses at the end of a stroke, including the masses of the piston and rod, the load being moved, and the fluid medium operating the cylinder. When the cushion operates, the additional thrust is imposed on the cylinder assembly and it will change length. What about the fluid conductors?

Consider protecting exposed rods from abrasion and corrosion that could destroy the rod surface and, in turn, the rod seal. In especially dirty environments, protect the rod with a cover such as a rod boot or bellows.

Operation conditions

1. Load and Force Requirements:Determine the required force or load capacity of the hydraulic cylinder based on the application. Ensure that the selected cylinder is capable of handling the anticipated loads.
2. Cylinder Type:Choose the appropriate type of hydraulic cylinder for the application. Common types include single-acting, double-acting, telescopic, and differential cylinders. The selection depends on factors such as load direction, speed, and space constraints.
3. Operating Pressure:Ensure that the hydraulic cylinder is operated within its specified pressure limits. Exceeding the recommended pressure can lead to cylinder failure and pose safety risks.
4. Speed of Operation:Consider the speed requirements of the application. Hydraulic cylinders have different speed capabilities, and the system should be designed to achieve the desired speed without compromising safety.
5. Mounting Orientation:Take into account the mounting orientation of the hydraulic cylinder. Some cylinders are designed for specific mounting positions, and improper orientation can affect performance and longevity.
6. Environmental Conditions:Consider the environmental conditions in which the hydraulic cylinder will operate. Factors such as temperature, humidity, and exposure to contaminants can impact the performance and longevity of the cylinder.
7. Cylinder Material and Coating:Select the appropriate material for the hydraulic cylinder based on the application and environmental conditions. Consider coatings or treatments to protect against corrosion and wear.
8. Seal Selection:Choose the right seals for the hydraulic cylinder to prevent fluid leakage. Seals should be compatible with the hydraulic fluid used and capable of withstanding the operating conditions.
9. Maintenance Requirements:Understand the maintenance requirements of the hydraulic cylinder. Regular inspection, lubrication, and replacement of seals are essential for prolonged service life.
10. Safety Measures:Implement safety features such as pressure relief valves and flow control valves to prevent overloading and ensure controlled movement.
11. Fluid Compatibility:Ensure that the hydraulic fluid used is compatible with the cylinder's seals and materials. Contaminated or incompatible fluids can lead to system failure.
12. Installation and Alignment:Follow proper installation procedures, including aligning the hydraulic cylinder correctly. Misalignment can lead to premature wear and reduced performance.
13. Control System:Choose an appropriate control system for the hydraulic cylinder, considering factors such as manual control, automated control, or integration into a larger hydraulic system.
14. Regulatory Compliance:Ensure that the hydraulic system and components comply with relevant industry standards and regulations.

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