A specially designed trenching excavator was built to dig a drainage line and lay plastic drainage tile at the same time. The drainage tile was coiled up on the back of the trencher, conveyed over the top of the machine, and placed into the freshly dug trench by a bucket. A trailing plow would push the dirt back into the trench, covering up the tile. The special design allowed a single machine to perform all of these operations at once. As a result, farmers could begin planting as much as four weeks earlier than when using up to three conventional pieces of equipment.
Workers would erect a single pole with a laser that rotated 360 deg., and a laser receiver on the trenching excavator positioned the depth of the trench. The laser output was tilted at an angle so that the drainage tile would be laid with a slight pitch to direct early spring moisture away from the fields.
The hydraulic system was controlled with proportional valves to maintain the proper depth of the bucket and plow. The system’s pump was located below the reservoir, with the case drain routed up to the top and into the reservoir. A standard filler breather incorporated a 25-µm filter element. The pump intake line, which came out from the bottom of the reservoir, had a ball valve shutoff.
The excavator was stored in a heated barn in the winter, and it seemed to work well even in zero-degree weather. Every so often, though, the shaft seal would blow out after the machine was started in cold weather. Workers took the machine back into the barn to remove the pump, only to find it in great condition. Before re-installing the repaired pump, they blew compressed air into the case drain line, which seemed free of any restrictions. They were using a low-viscosity hydraulic oil formulated for use at low temperatures. Workers did notice that the problem only occurred in the cold weather when the machine had been left outside overnight.
Any idea what was causing the problem?
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Solution to Last Month’s Challenge:
Charged accumulators can produce very high flow for short periods if it is not controlled. If a control orifice in a pilot-operated relief valve becomes plugged, the relief will fully open at low pressure, opening a path for the accumulators to dump uncontrolled flow back to the reservoir.
The reservoir’s air breather has a limited flow capacity for releasing air to the atmosphere. The breather could not keep up with the sudden surge of flow from the accumulators, so air pressure rapidly increased inside the reservoir. The pressure acting on the relatively large surface area of the reservoir created a force strong enough to blow its doors off.