Hydraulic components are expensive and, so, regardless of the amount of spares that you carry, one thing you want to make sure of is that your inventory is not deteriorating in storage. With this in mind, here are a few pointers for the effective long-term storage of hydraulic components:
Store them indoors in a clean, dry area. This is stating the obvious and is easy enough to do for more compact components like pumps, motor and valves. But in the case of large or long cylinders, the warehouse department may find it more convenient to store them outdoors. This should be avoided if at all possible.
Protect all exposed metal parts. Smear the internal surfaces of cylinder eye/clevis bushes or bearings with grease. Pump and motor drive shafts should be wrapped in oil-impregnated tape such as Denso tape. The same goes for exposed chrome on cylinder rods. Before applying Denso tape or a similar product to a cylinder rod, make sure that the rod is fully retracted. If a product like Denso tape is applied to the rod when the rod is not fully retracted, subsequent retraction can result in damage to the rod seal.
Plug the component’s port connections with steel (not plastic) plugs or blanking plates. Plastic plugs don’t provide the same sealing integrity as their metal counterparts and are easily dislodged – both of which can result in contaminant ingression.
Consider filling the component with clean hydraulic oil through its service and/or case drain ports – particularly if it is an expensive, large-diameter or high-pressure pump, motor or cylinder. I say “consider” because there are a few issues to understand before doing this.
If the component – a cylinder, for example – is not filled with oil, it will obviously be filled with air. If this air is not perfectly dry, then as the ambient temperature decreases, the air can reach the dew point. This results in moisture forming on the inside of the cylinder tube. This can cause spot-rusting and pitting of the tube surface, which will reduce the volumetric efficiency of the cylinder and the service life of the piston seal.
Completely filling the component with clean hydraulic fluid prevents this from occurring; however, there is a major caution, which is best illustrated by an example. Say a cylinder is prepared for storage during the winter months. When the cylinder is filled with oil, the ambient temperature is 10 degrees Celsius. A year and a half later, during the middle of summer, the same cylinder is set down beside the machine to which it is to be installed. In the heat of the mid-day sun, the temperature of the cylinder rises to 40 degrees C. Assuming an infinitely stiff cylinder, the pressure of the oil in the cylinder resulting from the rise in temperature can be approximated by the formula:
p (bar) = 11.8 x ΔT
So, the theoretical pressure of the oil in the cylinder is now: 11.8 x (40 – 10) = 354 bar, or 5,134 psi! When it comes time for the unsuspecting mechanic to crack loose the blanks on the service ports … well, let’s just say that’s more excitement than he is expecting.
That said, cylinders (and other hydraulic components) CAN be safely filled with oil for storage provided that you:
The flip side to all of this is, if you get involved in installing hydraulic components, when it comes time to remove blanking plates or plugs, always assume the possibility that the component may contain oil under pressure. And, take the necessary precautions.
Another storage issue peculiar to large cylinders is the question of orientation – that long-term horizontal storage can permanently distort the seals due to the weight of the rod “resting” on them. Based on my experience, the argument that large hydraulic cylinders need to be stored vertically to prevent this is a myth. Two cases I was directly involved in come to mind. In both cases, the cylinders in question were off 400-ton, mining-size hydraulic excavators. We are talking about cylinders that weigh between two and three tons. The piston rod typically weighs more than a ton by itself.
These big, expensive, high-pressure cylinders were suffering premature seal failures. In both cases, the machine owners sought the advice of “seal experts”. Their recommendation was to store the cylinders vertically.
Let’s consider the reality of doing this:
Someone drops a three-ton cylinder with a closed length of four meters at your feet and tells you to store it vertically – so it doesn’t fall over and destroy itself or, worse still, kill someone. Not a five-minute job, but it’s possible, I suppose.
Next, a truck arrives to transport the cylinder to a remote mine site. The route consists of 1,000 miles of rough, unsealed road. Given that you have gone to the trouble of storing the cylinder vertically in the warehouse, surely you must insist that it is transported in the same orientation? I mean, if it can’t be stored horizontally in a shed, then surely the pounding it is going to get if it’s laid down on the back of a truck will turn the seals into mush, right? The truck driver thinks you’re crazy, but he doubles his rate and obliges anyway.
The cylinder arrives at the mine site in the mandated vertical position. Trouble is, it’s a “stick” cylinder, so it’s orientation on the machine is horizontal. If the bearing (wear) bands on the piston and in the gland can’t adequately support the piston rod and prevent it from distorting the seals when the cylinder is sitting in the shed or bouncing around on the back of a truck, how on earth will it cope with the thrust developed when it goes into service on a 400-ton excavator?
Common sense would tend to suggest that if the bearing bands have sufficient area and are correctly tolerenced to adequately support load-induced thrust without distorting the seals, then surely they will cope with the static weight of the piston rod in storage and any dynamic loading that may occur during transport?
Whether you agree with this assessment or not, troubleshooting is a process of elimination. And in both cases, when the seal failures continued to occur even after the cylinders had been stored vertically, it became clear to all concerned that this was not the root cause of the problem. No surprise to me. That said, if your spare hydraulic cylinders can be stored vertically – safely and inexpensively – then there’s certainly no harm in doing so. On the other hand, think twice before going to a lot of effort and expense to facilitate this.
In the final analysis, taking proper care of your spare hydraulic components is very similar to caring for the ones on your machines: Keep them clean, dry and cool!