I am a novice to hydrostatically driven vehicles, but shouldn't the oil volume be at least 1-2 times the maximum pump volume? For instance a 7 gal/ min pump is best fitted with a 10 gallon tank. I recently shopped (and bought)a Zero turn mower. The two pumps are each 1.6 gal/min but the hydraulic tank holds only 3 quarts. Oddly enough there is plenty of room for a bigger tank. Should I replace the tank with a bigger one?

The reservoir volume of any machine is dictated by several criteria.
1. Needs enough volume to account for heat induced fluid expansion and un-equal area cylinder extension without uncovering the pump inlet.
2. Trapped air can separate from the oil while stagnant, though this is typically a slow process.
3. Metal reservoirs do remove some heat from the oil (though it is not nearly as effective as a oil/air or oil/water cooler)
4. More oil should help lengthen the change interval, provided it doesn't become contaminated or burnt.

I have run reservoirs that had turnover times as short as 15 seconds. In practice the longer the better - to a point. Conventional wisdom is turnover times of 2-4 minutes. Larger reservoirs add weight and cost.

One other issue with a larger reservoir for your machine in particular is that a larger reservoir will allow the oil to slosh around much more than the smaller one you have now. You could easily get into trouble with oil bypassing the breather.

I would leave it alone least you void your warranty.

ISZ

Take a look at this catalog for Eaton's small ZTR Hydra-Static drives. Notice the reservoir on them. http://hydraulics.eaton.com/pr...s/E-TRLD-TM001-E.pdf
The literature claims the tank here is for EXPANSION due to heating only. The oil in the Pump/Motor circuit continually circulates at the amount of flow and pressure to overcome the Torque requirement at any moment, so energy losses are low and heat generation is only from the Efficiency losses of the motor bypass oil.
Notice the Cooling fins on the Pump and Motor bodies and the recommended cooling fan shown on the Pump Input Shaft end.
Except for mounting location heat sources, all heat in a hydraulic circuit is produced wasted energy and a Hydra-Static drives only energy waste is from the Ineffiiencies from oil that is pressurized and allowed to bypass through internal clearances.

Nicely explained. Thank you both. This sight is so cool.

Dear Duane:
The rule of thumb for sizing a reservoir for an open-loop hydraulic system is the oil capacity should be three to five times pump flow. Additional factors need to be considered i.e., cylinder displacement, volume occupied by system, physical reservoir size limitations (mobile equipment), weight limitations (mobile equipment), etc.
An advantage of a hydrostatic tranmsission is that the reservoir is sized for charge pump flow rather than main pump flow. The charge pump flow can be estimated at 10% to 15% of main pump flow, or you can simply refer to the pump manufacturer's specifications.
The pump manufacturer's product engineering department is also standing by to offer assistance.
Hope this helps.
Regards,
Rory S. McLaren

Rory wrote:

quote:

The pump manufacturer's product engineering department is also standing by to offer assistance.

I think it's sad that one answer to a generic question like Hydraulic Tank Size is as above.

Even more sad is the fact that there is so much conflicting information on a regimen that has been around so long. I believe a question, on application, to {6} different manufactures of any type Fluid Power equipment, would result in at least {3} different and possibly conflicting bits of information. At least I see that in many of the Hydraulic and Pneumatic training offerings and lot of answers on the 4-5 Fluid Power Forums on the Web.

Also, I have had feedback from students that questioned why I made conflicting statements about a particular area of Fluid Power. They asked since another Instructor had offered different instruction/answer that made them wonder who was right.

Should not be that way for a discipline as old as Hydraulics and Pneumatics. But, it is common for even simple questoions to have answers that are different and even in opposition.

On matters of basic physics the answers best converge to one correct answer, as our best understanding anyway.

But many times the applications are way different and the answers are different.

as bud points out, often the solution proposed for minimum price and size and complexity is an energy hog. For something that cycles once a year, who cares. But for an all day operation, the more expensive different approach may have totally different decisions.

Industrials vs. mobile also way different 'rules of thumb'. And rules of thumb were often generated so that persons without much TRAINING could throw stuff together and it would usually work. With proper skill and training, they can evaluate whether the rule is still valid.

Example: tank size of 1 to 3 x pump flow was valid for fixed pumps, cooling, settling, deaeration, fluid storage, thermal and displaced rod volume, etc. Might be spacd and volume and weight and cost overkill, but things should work. With the knowledge to thoroughly understand the application, a person may meet those needs in other ways and go with a tank a tiny fraction of 'the rule'.

So in one sense the varying answers may be from wrong understanding (curable ignorance), or from dumb answers (incurable stupidity), or from legitimate design decisions based on the application and knowledge of the designer.

Knowedge and training........

kcj