APPLICATION SITUATIONS FOR HYDRAULIC CYLINDERS
The typical stroke ranges of AHP Merkle hydraulic cylinders extend from 1 mm to 2,000 mm (0.039 to 78.74 inches). Naturally there are also special designs with longer strokes. In the determination/dimensioning, particular attention should be paid to important operating conditions such as dynamics, piston speed, force ratios, etc.
During stamping, for example, very high dynamic loads are generated (switching impacts, pressure spikes), and both the cylinder and the seals have to be designed for them. Thus the guides are reinforced, the seals are adapted and the overall design is dimensioned for the significantly higher loads. Another difference between stamping cylinders and block cylinders is the larger ports, which are used to achieve higher flow rates.
HIGH PISTON SPEEDS AND / OR LARGE MASSES
With high piston speed and large moving masses, particular attention has to be paid to the approach to the end position. To avoid unnecessary impact loads, the use of hydraulic cylinders with end of stroke cushioning is recommended, or else the use of external shock absorbers – or even both. This always applied when the piston moves to the end position at a speed greater than 0.1 m/s (0.328 feet/s).
An important factor in deciding on end of stroke cushioning or external shock absorbers is not just the moving mass, but also the stroke. If the stroke is very short, the cushioning can have a strong effect on the cylinder motion, thus making it „sluggish“. In this case it is advisable to use external cushioning.
The greater the piston speed or the mass moved by the cylinder, the more important cushioning is.
Quite often in mechanical constructions transverse forces are generated; these must never be absorbed by the hydraulic cylinder (see also DIN EN ISO 4413). For one thing, this would damage the guides and seals, and secondly the piston rod can undergo plastic deformation if too much force is applied. For this reason it is necessary to use suitable guides to absorb the transverse forces that are generated; such guides are standard, for example, in the push units and core pull units from AHP Merkle.
Furthermore it is possible to prevent the undesirable application of force to hydraulic cylinders by means of suitable couplings and pivots.
If transverse forces are not absorbed completely by appropriate design elements, there is a risk of damage to the guides, running surfaces, seals and to the piston rod.
In order to operate several cylinders (even identical ones) synchronously in an application, there are certain special considerations that must be kept in mind. This is because the synchronous running of several axes (and this also applied to hydraulic cylinders) can only be achieved with additional design measures, such as precise, stable guides. The reason for this is the large number of physical parameters affecting the system. For hydraulic cylinders this means that one of the cylinders always has the lowest resistance, and thus even units with identical designs do not always advance and retract completely identically. If synchronous applications are operated without the appropriate design measures for synchronization, damage to the cylinders can occur, and other elements in the system may also be damaged.
One effective way to achieve fault-free synchronous running is to use commercially-available flow dividers or flow splitters. These divide the available oil evenly among the cylinders. In addition, the pipes for the volume flow supply to the individual cylinders must have the same length (synchronous pipe system) and the cross- section of the pipes must be sufficiently large. In addition, external guides of an especially precise and stable construction are required. In most cases, a synchronous pipe system with a well thought out guidance of the molded parts to be moved is sufficient for many applications.
Another means of achieving synchronization is axis synchronization using a linear position transducer. Systems that are controlled in this manner offer the most accurate synchronization for it implementation of synchronous applications. Here proportional valves, control valves or servo valve perform precise control of the flow rate – and thus of the cylinder motion. However, the control electronics for this are much more complicated to implement.
Due to the complexity of synchronous applications and the resulting effects on the cylinder, overall construction and/or machine, AHP Merkle recommends performing a thorough investigation with regard to the force ratios, axis motions and other design details of the planned synchronous application.
UNDESIRABLE TRANSMISSION OF PRESSURE
If hydraulic cylinders are combined with each other to optimize motion profiles or the development of force, the possible effects must be monitored carefully and taken into account in the design.
Example 1 (coupled cylinders):
If two hydraulic cylinders coupled on the piston rod have differing piston diameters, the pressure in the smaller one (p1, A1) increases significantly when the larger one (p2, A2) „pushes“. This situation follows the following relationship:
With an output pressure of 250 bar (3625 PSI) and piston diameters of 50 mm (1 .97 inches) (large cylinder) and 32 mm (1 .26 inches) (small cylinder), the chamber pressure in the small cylinder increases to about 610 bar (8845 PSI) . With an even smaller piston diameter of 25 mm (0 .98 inches) (small cylinder) the value in the cylinder chamber even increases to 800 bar (11,600 PSI).
If in this arrangement the large hydraulic cylinder does not push on the piston surface, but rather on the ring surface of the smaller hydraulic cylinder, the increase in pressure becomes even more dramatic.
Example 2 (external forces):
One typical source of risks is when large external forces act on hydraulic cylinders. Such situations can occur, for example, when the valve for retracting the ejector does not open at the right time. The large force generated over the large surface of the main cylinder is then transmitted to the small surface of the ejector, creating a tremendous force that „blows up“ the hydraulic cylinder.
PUSHING LOAD / BUCKLING STRENGTH
When designing hydraulic cylinders it is especially important whether the cylinder is pulling or pushing, or if they have to apply force in both directions . In the case of pushing loads, the buckling strength of the piston rod has to be taken into account. This is especially true for long strokes.
The buckling strength of the piston rod is influenced by the following factors:
- Diameter of the piston rod
- Length of the piston rod / of the cylinder
- Fastening of the piston rod and of the cylinder
At www.ahp.de there is an interactive calculation tool for the proper design, dimensioning and selection of hydraulic cylinders. The design tool ahp.calc (app) can be used to carry out a lot of complicated alculations in an easy and user-friendly way.
As a special design it is also possible to provide an additional leakage oil connection in the hydraulic cylinder. This is always required if no microfilm on the piston rod is permissible, such as in the food industry, for example. In this case there must be an additionally sealed annular chamber. The oil from the lubricating film can be deposited there, from which it is removed via an additional connection. This design measure has also proven useful to prevent hydraulic fluid from escaping into the environment even if the sealing capability of the rod seals is degraded due to normal wear.
Generally it is assumed that hydraulic fluids are non-compressible. In fact, however, in practice an appreciable „compression“ of the fluid is noted at high pressure loads. This type of „negative expansion“ is naturally also transmitted to the piston rod, which leads to undesirablae changes in the positioning of the piston rod and in the stroke motion actually executed by the piston rod.
A cylinder with a piston diameter of 100 mm (3.94 inches) and a stroke of 100 mm (3.94 inches) can settle by about 1.5 mm (0.059 inches) when the load changes from 0 kN to 157 kN (0 to 17.65 ton-force) (corresponds to a pressure change of approx. 200 bar (2900 PSI)). At 500 bar (7250 PSI) such a „compression“ has already reached a value of 3.75 mm (0.15 inches).
This example does not take into consideration, however, either the seal effects or the feedback from the overall design of the hydraulic system, for example the use of hydraulic hoses.