Using infrared thermography when designing and specifying hydraulic power units can reveal design areas that need attention and improve performance of a manufacturing operation.
By Deborah Hays,
DMAX Ltd., Dayton, Ohio
|Figure 1. FLIR P Series IR camera used by predictive technology team features an LCD monitor to show heat signatures in real time. |
In specifying a transfer machine for a cylinder block machining line at General Motors’ Duramax Diesel engine plant, designers at DMAX Ltd. assumed the hydraulic power unit would be adequately sized for the job. Initially it was. However, engineering design margins and the possibility of future expansions were overlooked. So when a power unit kept overheating and shutting down the cylinder block line, they needed to solve the problem. The immediate fix turned out to be surprisingly simple, and was discovered by using an infrared (IR) thermography camera to reveal some fundamentals of heat transfer and engineering design.
Looking for trouble
The predictive technology team is always looking for trouble — not with people, but in equipment. The team includes a mechanical technician, electrical technician, and an IR camera to detect thermal problems. This approach to maintenance is based on the principle that many components heat up before they fail. Because all objects emit thermal radiation in the infrared spectrum, their heat signatures can be seen by an IR camera and displayed on a monitor in real time, as seen in Figure 1. This non-contact temperature data can be also be downloaded to a digital storage device for later analysis.
|Figure 2. IR camera thermogram/photo pair of the overheated hydraulic power unit reservoir. |
The IR camera is primarily used to run predictive maintenance routes on electrical panels, bus plugs, motors, and other mechanical components. However, on some occasions, a team is called to the plant floor to help troubleshoot a problem. A recent call involved a hydraulic system that is part of a transfer line that machines the engine block. The transfer machine has five cutting stations, two rotate stations, a gauge station, check station, and five idle stations. Hydraulic cylinders are used to transfer the blocks and rotate them so the part can be clamped when it is being machined. Hydraulic cylinders are also used in several stations to raise and lower large cutting heads.
The hydraulic system reservoir has a volume of 600 l, and zinc dichromate coated steel tubing distributes fluid to hydraulic components. Several supply and return lines run to each station as well as to the large hydraulic cylinders. The transfer machine’s original manufacturer specified that the maximum system temperature should be no higher than 120°F when running at a normal pressure of 1200 psig. The system has a high temperature fault set at 117°F.
Electricians from the cylinder block line were being called repeatedly to this operation to reset high temperature faults on the hydraulic system. They finally asked mechanics to check the system for abnormalities. All pressures and flows were found to be within original specifications. The next step was to verify that the actual temperature in the hydraulic system matched the thermocouple reading. The team checked the temperature using a FLIR P Series IR camera and determined that the thermocouples where operating correctly. As shown by the thermogram temperature scale in Figure 2 (left view), the power unit was operating near its 117°F fault setting.
|Figure 3. Chart, thermogram, and photo show emissivity and temperatures of steel tubing, without and with tape applied (red arrows). |
Keeping their cool — for now
Keeping production going is paramount, but electricians can’t afford to spend their day resetting temperature faults, so they needed a quick solution. Although the temptation was great, they were advised not to increase the power unit’s high temperature fault setting. Running a system with excessive heat causes the hydraulic oil to deteriorate rapidly, possibly causing a failure in the hydraulic pump, cylinders, or valves. Without time to carry out a root cause analysis, they simply used a fan to cool the reservoir.
Convective air flow can be an effective cooling medium. Although a large fan is a quick fix, it shouldn’t be viewed as permanent. It disrupts efficient movement of personnel and materials and poses a safety hazard. It also increases energy consumption.
Getting to the root of the problem
With the issue temporarily resolved, the team could focus its attention on finding the root cause of and permanent solution to the problem. After analysis, it was discovered that the hydraulic system was slightly undersized for this particular application. A gauge station subsequently had been added to the machine, increasing the system load.
Another factor contributing to the problem was the ambient temperature around the transfer line. On days when it was relatively cool, no temperature faults occurred. The most frequent faults occurred on warm summer days. Although the plant has a chiller system designed to keep the general plant temperature at a constant 76°F, the area around the cylinder block line can reach 90°F in the middle of summer.
An easy, cost-effective fix
Economics are always a factor in situations like these. It was decided not to spend money to upgrade a hydraulic system that worked fine 10 months of the year. The purchase of a chiller unit for the hydraulic system was also ruled out for the same reason. The fan — while effective enough — was in the way and was not really keeping the power unit cool enough. Although less frequent, high-temperature faults were still occurring. A permanent, costeffective solution was required.
A previous FLIR thermography training session covered the physics of thermal radiation, which relates to heat transfer. The total radiation from an object can be expressed as E + R + T = 1, where E is the emissivity of the object, R is the reflection of heat impinging on the object, and T is the transmission of heat through the object. For objects that are opaque to IR energy, T = O, and the equation can be simplified to R = 1 - E.
|Figure 4. Hydraulic lines before (top) and after (above) painting. |
This same training session introduced a thermographer who observed an overheated motor that had a stainless steel housing. He wanted to check it regularly, but couldn’t easily locate it among many other similar motors. He decided to paint the motor so it would stand out. Much to his surprise, this stopped the motor from overheating. Appa r ent ly, the paint reduced its reflectance and increased its emissivity, allowing heat to dissipate better, instead of being trapped inside.
To determine if this would wor k on the zinc di-chromate coated tubing, the team put electrical tape on two of the hydraulic lines and checked the temperature difference between the taped surface and the steel surface. A 4°C temperature change was recorded. Figure 3 shows the emissivity and temperatures of different hydraulic lines and locations on those lines, with and without the tape. Sp1 is an untaped area, Sp2 a taped area, and Dt1 the difference between those two temperatures.
To make this more permanent, the tubing was painted with the hope it would radiate enough heat to keep the hydraulic power unit running below the OEM fault temperature year round. Because the rest of the equipment is painted white, flat white spray paint was used on the hydraulic tubing. Figure 4 shows before and after photos.
Twelve cans of spray paint and a week later, the hydraulic unit was rechecked with the IR camera, with the results seen in Figure 5. The overall system temperature was reduced by 10°F, going from 117°F to 107°F. This is enough to keep the unit running even on the hottest summer days.
|Figure 5. Reservoir thermograms, before and after painting tubing of the hydraulic system. The darker color in the view on the right indicates a lower temperature (see temperature scale). |
Changing the spec
After the painting was finished, it was determined that the large surface area of hydraulic tubing in the system was why a 10°F reduction was achieved. Although this may not seem like much, sometimes it’s just enough to avoid problems.
To take advantage of this radiating surface, the specifications for purchased hydraulic systems were revised. From now on, all hydraulic lines on purchased equipment must be painted flat white. Another thought is to consider specifying an engineering design margin and the possibility of future expansions in the sizing of hydraulic power units.
Deborah Hays is a Level II thermographer and mechanical service technician for DMAX Ltd. Contact her at firstname.lastname@example.org or (937) 425-9323. For information on IR cameras and criteria for purchasing one for manufacturing/maintenance applications, visit www.goinfrared.com/manufacturing.