Hydraulic cylinders are typically designed for a long and arduous service life. But while quality materials and accurate manufacturing processes do support this, like with any assembly with moving parts, wear does occur. During difficult economic times, however, servicing, repair and overhaul may be preferable to the replacement of complete cylinders. Andy Lindgren from Parker Hannifin comments
When choosing a cylinder it is important to consider the whole-life cost benefit of a maintainable design. While a simple tie rod cylinder may cost a few hundred pounds, more elaborate electro-hydraulic cylinders can cost many thousands. Therefore the selection of a quality cylinder offering extended life is important, and the benefits of replacing components rather than complete cylinder replacement are considerable. Correct cylinder selection and careful installation are also important to ensure that wear rates are not accelerated.
Glands and seals
If a cylinder develops a leak, it is most likely to be from the gland. Premature seal failure may be caused by the presence of abrasive particles – either from the external environment or from contaminated hydraulic fluids. Seals can also fail due to excessive temperatures, materials which are unsuitable for the application, or through premature wear caused by excessive side loading.
The use of separate, detachable gland assemblies in cylinder designs can help facilitate ease of maintenance – often without the need to remove the cylinder from the machine. Detachable glands can also simplify handling and reduce the stock holding of spare parts.
The piston rod has a dual function. It transmits the force generated by the fluid acting against the piston, and it guides the direction in which that force is applied. This second function leads to one of the most common causes of early seal failure – if the piston rod and the machine member to which it is attached are not precisely aligned, side loading of the piston rod will cause rapid wear to one side of the gland and seal assembly. Not only is the efficiency of the cylinder reduced, but the condition may also result in contamination of the hydraulic fluid, leakage and, potentially, rod breakage.
Specifying cylinders that use high quality materials and finishes can offset higher initial costs by giving a reduced total cost of ownership. High tensile, carbon alloy steel piston rods that have been subjected to hardening, plating and polishing processes will not only maintain sustained high levels of performance, but also increase seal life and reduce overall maintenance requirements.
An hydraulic cylinder is at its most efficient when 100% of the available hydraulic pressure is applied to the surface of the piston. Any leakage
of pressurised fluid past the piston seals will not only result in a rapid drop in performance but it also means that the cylinder will not support a suspended load, which may be a safety critical requirement in applications such as presses.
Piston seals and wear rings such as those used in Parker’s HMI series cylinder range offer high dynamic performance and durability that ensures sustained high productivity and minimised downtime.
In the face of continually increasing pressure to keep manufacturing costs low, replacing individual components rather than the complete cylinder is a solution. While kits of replacement gland seals are available, best practice would recommend the installation of complete glands with the seals pre-loaded. This helps minimise downtime, restores alignment where wear has occurred and significantly reduces the risk of damaging a seal while fitting – a particular issue in the case of more difficult to handle and delicate small diameter seals.
Body and end caps
The cylinder body and end caps together contain the full hydraulic pressure created by both the hydraulic system and the load under dynamic conditions. They are designed to with stand peak pressures which can exceed twice the maximum rated operating system pressure.
As an example, Parker’s tie rod cylinders, which are designed to be fatigue-free at the maximum rated pressure and have a 4:1 factor of safety, are designed to ensure the safety of personnel as well as the long working life of the cylinder. Using high grade tube and tie rod materials, and machining end caps from solid billets, results in strong dependable cylinders that deliver high productivity over a long service life.
Careful attention at the design stage to considerations such as cylinder mounting will also have a bearing on the ultimate cost of ownership. Cylinder mountings transmit the force applied by the cylinder to the surface on which it is mounted. They are responsible for controlling alignment and resisting movement which would result in eventual fatigue failure.
The most efficient transmission of force occurs along the cylinder’s centre line, as with flange-mounted cylinders. Trunnion mounted cylinders also absorb force on their centreline, but lack the rigidity of a fixed mounting. Foot-mounted cylinders do not absorb force on their centrelines and should be firmly secured, using a thrust key for positive location where necessary.
The load attached to an unregulated cylinder stops abruptly when it meets a physical obstacle or when the piston in the cylinder reaches the end of its travel. The resulting shock is transmitted to the machine and/or cylinder, often with considerable force and noise.
In some simple applications, this is acceptable. In others – such as in assembly automation – more controlled acceleration and deceleration is required if shortened service life and compromised productivity are to be avoided. By utilising the rising back pressure in the cylinder as it approaches the end of its stroke, system fluid can be used to create a progressive retarding effect as flow is reduced using a tapered ‘cushion’ valve.
A primary consideration
Lower cost of ownership rather than initial cost should be a primary consideration for design engineers when selecting hydraulic cylinders for their applications. Quality materials and sophisticated design and manufacturing processes by cylinder manufacturers may not always result in the lowest cost cylinder. If the need to carry out repairs or unscheduled maintenance can be avoided, however, then the high costs of downtime can quickly be offset.