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.
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.
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.
Proper installation begins with machine layout; here are some rules:
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.
Category: Seals
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Piston seals are used to create a seal between the piston and the cylinder bore, preventing leakage of hydraulic fluid or air. This seal is internal to the cylinder and allows pressure to be maintained on one side of the piston, creating the force to move it back and forth. In this blog, Martin&#;s Rubber Company explains the function and advantages of single and double acting piston seals.
A piston seal works by containing pressure on one side of a piston without leakage. This allows maximum mechanical effort to be applied to moving the piston along the bore of a cylinder. A piston seal can be driven either hydraulically or pneumatically, with an appropriate piston seal design for each system and application.
Piston seals are intended to prevent leakage past the piston and therefore maximise the efficiency of a system. Pressure can either be applied in one direction only (single acting) or from either direction (double acting) to drive the ram to which the piston is fixed both in and out.
Single acting piston seals contain pressure on one side of a piston without leakage. This allows maximum effort to be applied to moving the piston along the bore of a cylinder. Single acting piston seals only contain pressure on one side of the piston. The seal is required to retain pressure from that direction, which then moves the piston along the cylinder in a &#;single action&#; &#; hence the name.
It is worth noting that the piston in single-acting cylinders may have oil on the pressure side only, with the opposite sidefilled with air. Therefore, piston seals should leave a minimum of oil film when passing along the cylinder bore. The transportation of oil would otherwise result in a leakage. The air side of single acting cylinders may also push air out and draw air in as the piston reciprocates. This air may carry moisture and contaminants into the cylinder, which can also lead to seal damage.
Double acting piston seals contain pressure on both sides of a piston, which is applied from either direction. This drives the ram that the piston is fixed to both in and out or stops it in a controlled position. They have a symmetrical cross section and identical sealing functions in both directions. Double acting piston seals normally consist of a slide ring and an energiser. The deformation of the elastomeric energiser provides enough force to keep the slide ring in dynamic sealing contact with the cylinder bore, while also sealing statically against the seal housing groove.
In the case of a double acting cylinder, a relatively thick lubrication film can be permitted between the piston seal and the cylinder bore to minimise friction and wear. However, the transportation of fluid occurring during dynamic operation is relatively insignificant in most applications.
The chosen piston seal material influences the choice of seal profile. Depending on the profile and the required characteristics, a piston seal can consist of one or several materials. Common piston seal materials include nitrile rubber (NBR), thermoplastic polyurethane (H-PU), acetal, Viton, EPDM and polytetrafluorethylene (PTFE). When determining the best seal design and material for a particular application, consideration also needs to be given to the pressures expected, as well as considering the acceptable friction losses and whether the piston head is integral or split.
Given the different functions of single and double acting seals, it is important to specify which seal type you require. Our single acting piston seals can be CNC machined to bespoke sizes. We can add material to compensate for worn or re-manufactured piston or cylinder conditions. Plus, we can help to restore the original performance of used equipment, as well as provide components for new build designs. View our standard profiles for general guidance or, better still, why not speak to one of our friendly and knowledgeable advisers for some professional guidance to get the most appropriate single or double acting piston seal for your application.
Find out more about our standard and bespoke Single and Double Acting Piston Seals, please call Martin&#;s Rubber Company today on 023 or [ protected].
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