The performance of Hydraulic valves is incredibly important to the safety and smooth running of machinery and operations worldwide. One of the main problems manufacturers face is leakage within their hydraulic valves.
Hydraulic spool valves rely on an extremely close fit between the spool and its bore, in order that leakage is minimised when the valve is shut off. Any leakage can cause a cylinder to shrink, and the circuit might even be under considerable load when ‘off’ or supporting a heavy item, and unlike a tap that completes a perfect seal, there is no element to compress and do so when it comes to hydraulic valves.
The process of yielding precision around the micron scale (0.001mm) is done by skillfully grinding the spool to size and honing the bore. It’s a very fine line of precision that needs to be followed in order to ensure proper performance: too great a clearance will cause excessive leakage, yet too little may cause the valve to jam under hot conditions.
In order to combat this, continuous measurement of the honing and grinding processes are essential. This is difficult enough when you consider that temperature variations, user technique, particles etc. can all skew the measurement to the point where it is ineffective for controlling the process, but even more so because of the extreme tolerances involved, making dimensional measurements of the components unreliable.
A good micrometre might have a resolution of 1 micron, but this is a large proportion of the spool tolerance band. Due to this, the best way to properly inspect spool valves is by directly measuring their leakage.
The finished spool and housing are placed in the shutoff position, and pressurised oil applied to one of the valve ports using a specialised test rig. Such a rig has a high-precision flowmeter to measure the very small leakage flows, typically less than 10 cc per minute.
We’ve recently designed a new leak test rig at HyPro, which represents an evolution of this technique, using modern technology to deliver various improvements.
So, how does this rig work, and why is it so much better?
How Has the Rig Improved?
Unlike our previous rig, which delivered the necessary pressure by running its pump against a relief valve, the new one has a variable speed pump that runs against a needle valve restriction. By changing the pump speed this enables testing at any specified pressure up to the maximum capability, should the need arise.
The benefits of this new variable speed pump include lower power consumption between the test runs of the rig, therefore restricting the likelihood of the oil overheating. Conversely, at the start of the test run, when filling the workpiece with oil, it runs at double speed for a second or so in order to do this more quickly.
The 1 kW Tec brand motor is controlled by an ACS355 drive from ABB, which accepts a single-phase supply and allows the rig to be powered from any standard 3-pin outlet. It powers a group-0 Vivoil gear pump supplied by MA Hydraulics, who also provided several other of the hydraulic components.
The rig’s sensors are of course essential to its function, and none more so than the flowmeter. We chose a GFM gear flowmeter by KEM-Kueppers, obtained via LitreMeter. This has both the necessary pressure rating and also outputs a huge 240,000 signal pulses per litre, giving the capability to measure flows down to 2 cc per minute.
Pressure and temperature are also important measurements; these are both handled by a combined sensor from Hydrotechnik, which is located close to the outlet spigot.
The most significant advance is the introduction of computer control over the whole test process. We are able to attain precise and flexible control of the testing pressure by controlling the motor drive using pressure feedback.
On the new rig, we’re also able to monitor real-time pressure and leakage through the colour screen display that traces a graph of the measurements. In doing so, it allows the necessary time for the air to purge from the workpiece and the leak rate to stabilise, before making the final measurement. This avoids having to retest due to uncertainty over the first measurement or waiting for a set time before assuming the flow had stabilised.
Another improvement to increase the accuracy of the measurements is that the computer is able to standardise the leakage measurement by accounting for temperature variations away from the specification. The viscosity of the oil, and therefore the leakage rate, varies considerably with temperature.
From knowledge of the oil’s viscosity index, a formula can infer the viscosity at any given temperature, and therefore the rig computes what the leakage would have been at the specified testing temperature. The standardised leakage measurement is compared against the tolerance band of the product type, which the user had selected from a list, and the rig then indicates a pass or fail result.
The microcomputer we used in this rig is a Raspberry Pi 3B+, with an official Raspberry 7-inch screen and a third-party ExpanderPi peripheral board for analogue interfaces. This computer can store its measurement data, filed by work order number and in a human-readable format suitable for importing into spreadsheets. The necessary control software was written in-house, using the Java language.
We built a further circuit board to perform various ancillary functions: sensor pre-amps; signal voltage conversions; a power transistor for the loading solenoid valve; and a hardware watchdog timer that will unload the hydraulic circuit in the event of a computer freeze.
Size and Integration with Current Machinery
Another improvement of the new test rig is its compact size, with a footprint of just 550 x 550 mm. This is of real benefit to us, as it enables the rig to fit into our machine shop where space is at a premium. In fact, the rig is located in the honing cell, whereas the previous one was in another workshop entirely.
The honing operator can now test up to 100% of spool valves in a cycle, with one valve on the test while our state-of-the-art Pemamo honing machine works on the next.
The operator knows quickly if the leakage rates are trending away from the ideal value, and will increment the honing parameters accordingly. Previously only a smaller proportion of valves could be brought to the old leak test rig, taking the operator away from honing work, and there was not the same rapid feedback.
Health and Safety Compliance
To maintain high levels of safety in a compact size, the rig has a clear polycarbonate guard lifting vertically on gas struts. The slender aluminium profiles for the guard frame, as well as the long-travel ball-bearing slides, were supplied by Hepco Motion. The guard has double interlock switches to cancel a running test and unload the hydraulic circuit immediately if it is opened.
Here at HyPro, we’re looking forward to seeing how these improvements can further revolutionise our production and manufacture process. If you’re interested, find out more about what we do at HyPro here.