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Low-pressure water hydraulics makes its move

Low-pressure water hydraulics can offer an alternative to both pneumatics and traditional water hydraulics. Recent tests show that cheaper LPWH components may now be within reach.

New is a relative term. In the computing world, 18 months seems to be a lifetime, or perhaps several lifetimes. In hydraulics, something first presented in 1997 can still freely use the term new, and not feel the slightest pangs of guilt. Such is the case with low-pressure water hydraulics. In practice, LPWH is a technology where tap water is used as a pressure medium and the maximum working pressure is under 50 bar.

The main benefits of LPWH are better motion control possibilities and higher forces than in pneumatics. On the other hand, the lower pressures compared to conventional water hydraulics reduces the costs of LPWH. The savings come mainly through materials. For example, low pressure enables the use of plastics and composites instead of stainless steel. Environments that require high hygiene level or fireproof solutions such as food processing, paper manufacturing, and the steel industry are potential applications for LPWH technology.

So, what stands in the way? Oddly enough, materials. The price of existing water hydraulic cylinders is quite high, mainly due to small production volumes and expensive materials. Stainless steel, traditionally used in water hydraulic cylinders, is expensive to purchase and machine. One possible solution is to replace the stainless steel with alternative materials. Test results of using a composite tube in a LPWH test cylinder with different seal materials are discussed here.

Figure 1. The Black Amalgon composite tube chosen for testing features a wide temperature range and a working pressure limit of 13.3 MPa.

The results illustrate that a composite tube can be used in a water hydraulic cylinder without any wearing problems. In addition, the measured friction values are acceptable and the efficiency of the test cylinder is competitive with a commercial water hydraulic cylinder. The best seal material was found to be ultra-high molecular weight polyethylene (UHMWPE), in this particular case.

Composite cylinder tube
Black Amalgon (BA) is a commercial composite tube. The tube is originally meant for pneumatic cylinders, but BA can be used in water hydraulic applications, as well.

The main specifications of the tube that was tested, Figure 1, were:
• bore: 32 mm,
• maximum pressure: 13.3 MPa (with tie rods),
• weight: 0.492 kg/m,
• operating temperatures: -180° to 135°C, and
• inner surface coated with vinyl ester resin gel.

The BA tube was measured before experimental tests. The coordinate measuring showed that the roundness of the tube was very good. The error was only 0.0063 mm. More inaccuracy was found in the tube's diameter, which was oversized by 0.030 mm. The tube's structural studies showed that the fibers were bound in a way that the tube tolerated high radial stress. The stiffness and surface roughness of the tube was also found to be good.

The measured average surface roughness values (Ra) were between 0.2 and 0.3 µm, the same as in honed stainless steel tubes. The vinyl ester resin gel on the inner surface of the tube is the reason for high smoothness. The outer surface of the tube is not coated and, therefore, slightly porous.

Figure 2. Test cylinder is designed for quick changeout of seals and tubes.

Material matters
The test cylinder, Figure 2, was designed for easy replacement of seals and tubes. Therefore, it was possible to test several seal and tube pairs easily with same cylinder parts. Materials used in the test cylinder were:

• aluminum bronze piston and cylinder ends,
• stainless steel cylinder rod chrome coating,
• stainless steel front plate, and
• steel tie-rods.

Figure 3. The piston seal chosen has a profile that is common in linear applications.

The seal type and material selection was a difficult task because there were not available references of similar cases. After trial and error, a very generally used seal type in linear movements, Figure 3, was tested. The piston structure was designed so only one pressure seal was needed, and it would be easily replaceable. The seal material selection was more difficult than seal geometry, due to the exotic sliding surface and lubricant. In total, three different seal materials were chosen for testing. First, the BA tube was tested with a PTFE (Teflon) seal. Teflon is the softest seal material and, therefore, if it would have worn the tube's surface, all other seal materials probably would have done the same. Carbon-Teflon (C+T) and UHMWPE were also selected because they are commonly used in water hydraulics.

In addition to the piston seal, two guide strips were attached to the piston to eliminate any side forces. The guide strips' material was pure Teflon, ensuring that the main stress to the tube's surface was caused only by piston seals. The cylinder was also designed for easy replacement of the rod seal and guide ring. This made it easy to compare friction measurements with different piston seal materials.

Friction and endurance tests

A friction test was arranged before the endurance testing. The aim of the test was to compare the friction and, furthermore, efficiency values of the test cylinder with different seal materials. In the beginning, both static and dynamic friction were measured without external load. After that, the dynamic friction was measured with constant loads and pressures and different piston velocities.

The dynamic friction measurement results were changed to efficiency format so results would be more understandable. The dynamic friction measurement equipment was originally meant for larger cylinders. Therefore, absolute accuracy was not high enough for analysis, especially at high velocities. However, the accuracy was good enough for studying possible differences between the tested seals.

The piston velocity and pressure varied between 150-200 mm/sec and 30 to 32 bar during the endurance test, depending on the motion direction. The load for the test cylinder was arranged by using pressure load instead of external load, which made the test installation simpler. External loading would have introduced some mechanical stresses, such as side forces. But in practice, these effects can be minimized by correct installation, so they were ignored. The condition of the piston seal in the endurance test was monitored indirectly through periodic leakage tests and by measuring the weight of the seal. Tube wear was monitored by measuring changes in tube's diameter and roundness.

Results from tests

Static and dynamic friction results were similar with every seal material. Therefore, it was not clear if one seal material was better than another. The average values were approximately 190 N for static and 155 N for dynamic friction.

The differences between seal materials were noticed more clearly in dynamic friction measurements. The measured efficiency curves were very similar with every seal material. The efficiency varied between 80 to 93%, depending on the load conditions, velocity, and supply pressure.

Friction is inherently higher with the lower supply pressures. Therefore, the efficiency is lower as well. The efficiency with a Teflon seal was highest. This is not surprising because pure Teflon is self-lubricating and, therefore, exhibits a very low friction. The UHMWPE seal had the second highest efficiency values but did not vary significantly from the C+T seal.

An endurance test was conducted to determine the durability of the BA tube and applicability of the test seals. The target of the test was 100,000 strokes — enough to clarify the wearing process. Tests began with Teflon, the softest seal material. Although Teflon worked well in friction tests, it is too soft and wore out quickly. Therefore, the test was stopped after 15,000 strokes and restarted with new seals and tube. The C+ T seal worked better, and achieved the full 100,000 strokes. Tests with the UHMWPE seal and a new tube also achieved 100,000 strokes.

The Teflon seal lost more than 8% of it's weight after only 15,000 strokes. Weight of the C+T seal decreased about 0.75% — remarkably better than pure Teflon. Wear of the seal affects the internal leakage. At worst, the internal leakage with C+T was about 2 ml/hr. This meant the piston could drift 2.5 mm/hr, which is not acceptable in most applications. The best results for endurance were achieved with the UHMWPE seal. Seal wear was so slight that it could not be reliably measured. Moreover, maximum internal leakage rate was only about 0.2 ml/hr.

Kari T. Koskinen and Matti Vilenius are with the Department of Intelligent Hydraulics and Automation at Finland's Tampere University of Technology. Max Lakkonen is with Hytar Oy Water Hydraulics, also in Tampere.

TAGS: Agricultural