The importance of small hydraulic pressure accumulators with some examples of their use

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01.02.2019

 

In the early days, before focusing on the production of precision rubber products, Hepako GmbH manufactured hydraulic accumulators and the accompanying rubber membranes indispensable for their function. At that time, Hans-Dieter Partosch, co-founder and managing director of Hepako GmbH for many years, published an article on the importance of small accumulators and examples of their application in the magazine Fluid - a German journal for hydraulics, pneumatics and component parts (edition Dec. 1979). Today's blog returns to this interesting and informative article, which has lost none of its relevance over the years. The original article is available as a download.

 

---- Hans Dieter Partosch ----

 

The use of small hydraulic pressure accumulators in technical devices

For many accumulator applications, only a small quantity of pressurized oil is required. The typical serial accumulator is installed separately and there are often space issues.  A manufacturer has specialized in small accumulators that can be integrated in systems and machines.

The standard uses of accumulators for volume compensation, shock absorption or pulsation damping are well known to hydraulic engineers. Their use is a matter both of cost effectiveness and technical necessity. For example, if a pressure accumulator is used to dampen pressure peaks in a piston pump, you have the benefit of extending the life of the pump by perhaps 30% to balance against the investment costs. An accumulator may be a technical necessity for potential emergencies, for example, in the event of a power failure to maintain functionality by using the stored energy to perform vital machine actions. For the manufacturer, it is important to produce low-priced, technically failsafe devices that comply with the accident prevention regulations.

Small accumulators as damping element

Figure 1: No chance of damaging vibrations when small accumulators are used as a damping element

Pressure accumulators as pulsation absorber

Figure 2: Pressure accumulators as a pulsation absorber extend the service life of piston pumps

[Hepako] has specialized in small pressure accumulators with a nominal capacity of 0.1 to 0.7 liters, made in stainless steel or a corrosion-protected version with welded or exchangeable membranes. The outlets either have internal or external threads and the gas valves VG8 screw threads. Customized connections are also an option.

There is thought-provoking potential for the manufacture of pressure accumulators according to customer specifications. In addition to manufacturing the accumulators, precisely manufactured molds and the ability to make customized rubber parts are important. The food and chemical industry, for example, requires stainless steel pressure accumulators and membranes made from special elastomers. It is possible to manufacture special membranes, costing relatively little, in a short time. The impermeability of the membrane for containment of different media can then be modified for the various specialist applications by experimenting with special compound mixtures and different ways of sealing to find the best solution for the particular application. 

Small pressure accumulator in hydraulic circuits

Figure 3: Standard nowadays - the use of small pressure accumulator in hydraulic circuits

Accumulator membrane manufacture

Figure 4: Accumulator membrane manufacture requires considerable attention to detail and experience

Makers of technical devices, tools and molds can integrate small accumulators into their existing systems according to the application. The accumulator can be designed to their specifications. The tool manufacturer can adapt the internal contours to the specifications of the manufacturer of the accumulator.

The components required for the assembly are obtained as standard components or as a custom design. The gas charging of the accumulator can be controlled at all times with an inflating and testing device and corrected as necessary. In the following are some examples of small pressure accumulators as standard devices and integrated custom devices in fixture construction or mold construction.  

Pressure oscillations and pulsations ...

... have a negative effect on more than just the service life of piston pumps. Measuring equipment and sensitive switching gear also suffer. In such cases, small pressure accumulators act as pulsation dampers and vibration absorbers. An accumulator can also reduce switching surges immediately before gate valves.

Tool breakage is the minimum damage ...

... for example, when a workpiece clamping is inadvertently loosened on a rotary transfer machine for processing machine parts (Fig. 5). The clamping system is supplied with oil from pressure line P, check valve 11 and the pressure reducing valve 10. The way valve 6 clamps and releases the workpiece in the clamping station. Continuous pressure is maintained and safeguarded in the processing station by check valve 7 and pressure accumulator 4. Even if the main pressure system fails momentarily, safe clamping will still be maintained for a length of time. Leakage of the oil distributor 3 supports an especially fast reduction in pressure reduction. Safety valve 8 and the shut-off valve 9 serve to discharge the pressure accumulator. The pressure switch 5 emits a signal for a drop in pressure.

Sealing fluid ...

... is essential for the correct function of the double-acting shaft seals in this example (Figure 6). It has the task of dissipating the frictional heat that is generated and avoiding penetration of the sealing gap by the product. The sealing fluid fills the space between the product-side and atmosphere-side of the  shaft seal set in a double-acting unit. Any fluid can be used as buffer medium, bearing in mind the corrosion resistance of all the parts it comes into contact with. The shaft seal 1 is supplied with sealing fluid via the power unit 2. The pressure relief valve 8 is responsible for correct pressurization of the unit. Thermometer 6 measures the exhaust temperature, and pressure gauge 7 measures the output pressure of the fluid, which is cooled by cooler 9 and cleaned by filter 10. If the drive unit fails due to a power failure or defect, the stored energy of accumulator 4 is maintained by the fluid pressure in the shaft seal and performance is guaranteed until corrective intervention can be undertaken. The pressure accumulator is protected by the safety valve 5.

Hydraulic pressure accumulator on a rotary transfer machine

Figure 5: A small hydraulic pressure accumulator serves as an energy reserve on a rotary transfer machine

Small accumulator as an energy reserve

Figure 6: The Burgmann shaft seals also include a small pressure accumulator as energy reserve

Rollers for tape recorders ...

… are manufactured using rubber injection molds (Fig. 8). The process involves the overmolding of a thin-walled metal bush e in the mold cavity f of the mold plate c with rubber material at 170° C via an injection channel.  The finished roller is pressed out of the mold cavity with the centering pin h after the injection process is complete and the top plate has lifted. To avoid damage to the thin-walled cylinder liner it has to be possible to set the ejector pressure to a certain value. A pressure accumulator b integrated into the base plate a with adjustable pressure piston k has the following advantages over a compression spring design:

·         precise pressure adjustment by means of a piston

·         pressure readings from the pressure gauge i

·         even tightly spaced load outlets can be connected to the pressure chamber j through bore holes

·         spring tension is retained even after a large number of load cycles

The seals and the accumulator membrane of the tool are resistant to a permanent operating temperature of up to 200° C.

Burgmann shaft seals

Figure 7: The Burgmann buffer fluid system with a small accumulator as an energy reserve

Accurate pressure adjustment in a rubber injection mold

Figure 8: The contact pressure of the thin-walled inserts can easily be adjusted in a rubber injection mold

Hydraulic clamping devices ...

... offer considerable advantages such as tension from one central point, minimal time requirement, positive control of the entire clamping process (no clamping operation can be forgotten), etc. The device shown in Fig. 9 for clamping thin-walled aluminum parts has an integrated pressure accumulator b in the base element a. By a right turn the piston h is moved into pressure chamber g and hydraulic fluid fed through drill holes in the device c. The clamping pistons e and d press the workpiece f on to the underlay - adjustment of the clamping piston diameter allows the force to be predetermined. The spring stiffness of the system is determined by the nominal volume of the accumulator. The clamping pressure can be controlled with a filling and testing device via the gas valve.

Small pressure accumulator in a clamping device

Figure 9: When space is at a minimum. Installation of a small pressure accumulator in a clamping device

Highest running accuracy through a hydraulic lathe mandrel and a combined small pressure accumulator

Figure 10: Highest running accuracy is achieved with a hydraulic lathe mandrel and a combined small pressure accumulator

High precision ...

... for high-quality turning, grinding and lapping work is achieved with a hydraulic lathe mandrel (type "Hofer") (Figure 10). The adaptor sleeve c sits firmly on the mandrel b, pressed on or welded, and depending on the clamping length has one or more pressure chambers d that are connected to the pressure chamber e through drill holes. When the piston f is moved into the pressure chamber e by rotation, the oil pressure in the system increases, the wall of the adapter sleeve c is uniformly elastically deformed and pressed against the positioned workpiece.

Uniform clamping pressure is achieved in conjunction with a pressure accumulator. Mandrels of different diameters can be flanged on the mounting flange a with a built-in accumulator g. Experiments with this system have shown that high torques can be transmitted over the whole length at low specific surface pressure.

 

--> The original article is available as a download here. 

 

If you have any questions or comments on this blog, or in future would like us to focus on a specific aspect of elastomers, feel free to contact us by email at info@hepako.de