Hydraulic fluid has a BIG job to do. It's a power transmission device, a lubricant, a heat transfer medium - and even a sealant, in some hydraulic components at least. And this is why I consider the fluid to be THE most important component of a hydraulic system. And certainly not something to be purchased on price alone.
But regardless of whether the hydraulic fluid you use is a synthetic, high VI, ashless, zinc-free, multigrade or monograde -- or any other of the many options available today, to do its job well, the hydraulic fluid needs help from its friends. The first of these, and possibly the hydraulic fluid's BEST friend, is the reservoir or tank.
Traditionally, recommended tank size for mineral hydraulic oil has been 3 to 5 times Q plus a 10 percent air cushion. Where Q is pump flow per minute - or mean pump flow per minute where a variable pump is used. For some special fluids, recommended tank size is even larger. For example, for hydraulic systems using HFC and HFD fluids, a tank volume of 5 to 8 times Q is recommended.
The thing is, the above formulas were not devised to sell more oil or to increase the extent of the spill risk. They were devised with hydraulic-system performance and reliability in mind. But these days, with increasing demand for lighter, more compact hydraulic equipment - particularly in mobile markets, tank oil-volumes of this order are becoming a thing of the past.
If tank oil-volume or more precisely, the lack of it, affects hydraulic system performance and reliability, then it follows that less than ideal tank volume compromises the hydraulic fluid. How? Well, in order to answer this question, the traditional functions of the hydraulic tank - and how these functions can (or can not) be subrogated to the hydraulic fluid's other 'friends' in the system - must be considered.
Beyond its most basic role of providing a store of fluid, the main functions of the hydraulic tank are to:
- dissipate heat; and
- allow contaminants to settle out of the oil.
In practice, the amount of heat dissipated from even a large tank is relatively small, so this function is easily and more efficiently subrogated to a heat exchanger. And when it comes to contaminants, the tank's role in settling out particles and water can be largely subrogated to the hydraulic system's filters.
This leaves one important function of the tank for which there is no clear substitute - other than adequate oil volume and therefore dwell time. And that is the release of entrained air. Air entrained in hydraulic fluid affects the performance and reliability of the hydraulic system in a number of ways, including:
- reduced bulk modulus, resulting in spongy operation and poor control system response;
- increased heat-load;
- reduced thermal conductivity;
- increased oxidation and thermal degradation (micro-dieseling) of the fluid;
- reduced fluid viscosity, which leaves critical surfaces vulnerable to wear;
- increased noise levels; and
- decreased system efficiency.
I've seen a lot of anecdotal evidence which suggests skimping on tank volume compromises hydraulic system reliability. One example that comes to mind is a hydraulic excavator manufacturer who, after increasing tank size and installed cooling capacity, saw typical pump life increase from 12,000 to 20,000 hours! This reinforces the point that no matter how good the hydraulic fluid is, it needs help from its friends.
Non-Conventional Tank Designs
In the past, when it comes to air release, there really hasn't been a substitute for the dwell time facilitated by sufficient oil volume. But these days, there are a number of alternative tank designs which promise large reductions in oil volume without compromising release of air from the oil. The cylindrical reservoir design is an example of these.
So if you design or build hydraulic equipment and you care about its reliability (and you should) don't skimp on tank volume, or be cheap about the tank's design and construction. If on the other hand, you service and maintain hydraulic equipment, there is little that can be done - economically at least, about installed tank volume - or the lack of it. But you can specify minimum required tank volume when ordering new equipment.
And don't forget, the tank, like the hydraulic system's heat exchanger and filters, must be taken care off. This involves regular drain-off of settled contaminants and occasional internal cleaning. Because to do its job effectively, the hydraulic fluid needs a lot of help from its friends.
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