Selectively pressurized tank for hydraulic fluid

A tank for oil or other hydraulic fluid is kept under a few pounds of air pressure during operation to keep air from being drawn into the fluid and to aid its flow to the pump. The pressure is provided dependably and quickly by a small air pump driven whenever the hydraulic pump is driven. A bleed-off discharge causes prompt dissipation of the pressure on shutdown so as not to cause oil leakage while the associated hydraulic pump is idle. The hydraulic level is below the pump shaft level so there will be no leakage by gravity.

BACKGROUND OF THE INVENTION 
The invention of which the present disclosure is offered for public 
dissemination in the event adequate patent protection is available relates 
to hydraulic reservoirs or storage tanks from which the hydraulic pump of 
a heavy-duty hydraulic-power equipment draws hydraulic fluid which is to 
be pumped at high pressure to operate the equipment. 
Modern usage of hydraulic circuits require special fluids for continued 
high performance, because of high speed and high pressures. This means 
that normal automobile type oils will not do the job. Automatic 
transmission fluids or its equivalent is required, even though the term 
"oil" is commonly used for them. This hydraulic fluid is not only very 
expensive, but hard to get in quantity, in out-of-the-way places. Hence, 
avoiding the loss of hydraulic fluid can be very important. 
One limiting factor in all hydraulic circuits is the ability of the pump to 
get the fluid through the suction line so as to be able to pump it under 
high pressure to the point of work. The temperature and the viscosity and 
length and size and friction of the line are the deciding factors. 
In order to help keep the pump charged, there has for many years been a 
well-known preference to place the hydraulic tank above the pump, or even 
to raise the hydraulic tank to the highest elevation that can reasonably 
be used. This gravity-aided feed, however, has a great disadvantage in 
that this head is always on the pump, and can, especially during an 
overnight stand, drain the tank when the pump's shaft seal leaks from wear 
or fatigue. The loss of oil is not always readily apparent, as the machine 
may be parked in a sandy area; and before it becomes apparent, the pump 
may cavitate, and destroy itself because of starvation or lack of oil. 
Another disadvantage of overhead oil tanks is that it is extremely 
difficult to change the seals of the pump, or even the pump itself. It is 
not permissible to provide a shut-off valve because of the danger of 
leaving it closed. Such a large vessel would be required to recover the 
oil that drains out of such a reservoir that recovery is rarely practical. 
Normal reservoirs contain a minute's supply of oil, so when this is 60 to 
100 or more gallons of oil, handling it during repair becomes a major 
problem. It is not easy to store and keep clean such a large vessel, even 
in a well-equipped repair shop. 
Loss of the hydraulic fluid from a tank can be a very serious problem 
except in the rare instances that a new supply is at hand. With nearly all 
machines, the engine direct-drives the pumps, with no drive-disconnect, so 
that the operator cannot even run the engines to get the machine to a 
repair shop without damaging the pumps because of not having them full of 
oil all the time. 
Because of these problems with a raised tank, an alternative for many years 
has been to locate the oil tank below the level of the pump, but provide 
pressure in the tank to give extra help in getting the fluid to the pump. 
The common way to provide the pressure has been by having the tank 
generate pressure within itself. This is accomplished by having a 
combination relief valve and check valve in the top of the tank so that 
level changes in the tank due to movement of the motor pistons causes the 
tank to trap the difference of the volume of air caused by this movement. 
Unused energy or friction of moving parts causes the trapped air to build 
up pressure more quickly by heating the air. Such pressure systems have 
also been used with elevated tanks, but even that did nothing toward 
removing the danger of loss of the fluid by an overnight leak. However, 
even with tanks lower than pumps, this system is not safe. 
The danger, then, is that because there is a pressure head at the pump, the 
same as with the overhead tank, loss of the fluid may result. Even the 
part of the pressure due to heating the air in the tank may long remain. 
Oil that is not moving, as when the machinery is shut down, loses its heat 
very slowly. 
The present invention removes the disadvantages of both systems by 
generating the necessary pressure, to help move the oil to the pump, only 
when the system is operating; by quickly removing the pressure when it is 
shut down; and by having the tank below the pump so there is no gravity 
head at any time. 
The invention also has the advantage, over pressure developed by trapping 
and heating the air in a tank, that the pressure is provided more quickly 
and minimizes the drawing of air into the fluid when the shaft seal of the 
pump is imperfect. There has long been recognition that with unpressured 
equipment, a failing shaft seal will cause air to be drawn in at the 
hydraulic pump. Air in the oil is quite objectionable, and there have been 
efforts through the years to avoid or minimize its being drawn in. So far 
as known, all such efforts in the past have failed to achieve the aim 
dependably, or have had objectionable side effects. 
SUMMARY OF THE INVENTION 
According to the present invention, a very small air pump is provided with 
its drive direct-connected so as to be driven whenever the hydraulic pump 
is driven. Its output flows into the storage tank above the oil to supply 
air pressure tending to force the oil to the hydraulic pump. This air 
pressure is preferably high enough to provide a slight positive oil 
pressure in the intake chamber of the hydraulic pump when the hydraulic 
pump is normally operating. The drive shaft of the hydraulic pump runs 
through a wall pump in or open to this chamber, and hence a positive 
pressure in this chamber precludes the drawing in of air along this shaft, 
even if the shaft seal is not in perfect condition. According to a 
preferred form of the present invention, a tiny bleed opening quickly 
dissipates the air pressure on shutdown, so that there is substantially no 
leakage after shutdown. 
The advantages and objects of the invention may be more apparent from the 
following description and from the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although the following disclosure offered for public dissemination is 
detailed to ensure adequacy and aid understanding, this is not intended to 
prejudice that purpose of a patent which is to cover each new inventive 
concept therein no matter how others may later disguise it by variations 
in form or additions or further improvements. The claims at the end hereof 
are intended as the chief aid toward this purpose, as it is these that 
meet the requirement of pointing out the parts, improvements, or 
combinations in which the inventive concepts are found. 
In some respects the tank 11 is conventional. Thus its outer structure is 
entirely of steel and it may have four vertical walls, a bottom wall and a 
top wall 24. One vertical wall 12 is provided with an outflow fitting 13 
and a return or inflow fitting 14. As indicated by diagrammatic line 16, 
the outflow fitting 13 is connected with filter means 17 through which oil 
is conventionally drawn from a little above the bottom of the tank. The 
top wall 24 of the tank is preferably permanently secured, as by welding. 
Access openings may be provided to permit periodic cleaning out of the 
metal particles which gather on the floor of the tank and the screens, 
such openings having pressure-proof closures. 
COOLING AND SETTLING PASSAGE OF ENT INVENTION 
According to the parent-patent invention, a curtain 21 rests on the tank 
bottom 22 and extends nearly all of the way around the periphery of the 
tank 11, being spaced only a short distance from the side walls 12 and 23. 
Although this curtain 21 should be secured to the bottom 22 (as by tack 
welding 25) and should extend above the expected maximum liquid level in 
the tank, it need not have a leak-proof seal to the bottom surface and 
need not extend to the top wall 24. When desired, oil which is returned to 
the tank 11 from conventional control valve 42 (which is conventionally a 
manually operated valve for controlling associated apparatus) is directed 
to flow through the cooling and settling channel 26 between wall 21 and 
walls 23 and 12 by a flap valve 27 extending from one side of a hub tube 
28 mounted on a shaft 29, to which it is firmly secured or keyed. After 
traversing the entire peripheral length of the cooling channel 26, to the 
discharge end 31 thereof, the oil flows into the main storage space 32 
comprising the entire tank volume inside of the curtain 21. 
In cold weather, when the oil needs to be heated by circulating it before 
starting hydraulic operations, the flap valve 27 is turned from the 
position shown in FIG. 3 to the position shown in FIG. 4, which is also 
the dotted line position of FIG. 1. This swinging of the valve is 
accomplished by a handle 33 secured on shaft 29 which extends to the top 
wall 24 of the tank. The shaft 29 also extends into a collar 34 welded to 
the bottom 22 of the tank. With this position of the flap valve 27, oil 
can flow directly into main storage chamber 32, as seen in FIG. 4. 
It is not necessary that the flap valve 27 seal tightly. In the cooling 
position it rests against a vertical jamb 36, which may be one angle of an 
angle bar welded to wall 12. In its other position it rests against the 
wall 23. 
Other cooling details and the cooling operation are described in the parent 
patent and need not be repeated here. An improvement which may now be 
preferred, however, is to be able to set the handle 33 for any desired 
apportioning of flow between the two entry routes, so that after the oil 
heats, a setting may be chosen that will keep the temperature within an 
optimum range. 
OIL PRESSURIZATION WITHOUT LEAKAGE 
The inventive feature to which the claims of the present application are 
directed relates to a problem which arises when the drive shaft of 
hydraulic pump 41 is at least as high as the oil level in tank 11. 
Although such a high location is desired, so that oil from the tank cannot 
leak by gravity through a defective seal of the pump drive shaft, it gives 
rise to the problem that the suction of pump 41 then tends to draw air 
into the system through the faulty seal. 
According to the present invention, the drawing of air into the oil is 
considerably reduced or virtually eliminated by pressurizing oil in the 
tank. Thus air pump 51 pumps air into airline 52 which discharges above 
the oil in tank 11. A relief valve 53 limits this air pressure to a 
desired value, such as 5 PSI. The pressure should be enough so that a 
positive pressure will be maintained at the intake or shaft chamber of oil 
pump 41 to prevent air from being drawn into the oil at this point, as is 
especially likely to occur as the shaft seal becomes worn. 
Oil in such tanks has been pressurized before, but the past pressurizing by 
using a sealed tank and relying on a temperature increase for pressurizing 
has defects. One defect is that the pressure increase is slow to develop. 
It is nonexistent when most needed because the oil is cold and more 
viscous. Later it may be inadequate. Another defect is that the pressure 
remains after the operation has ceased and may cause oil leakage. Gravity 
pressurization by having the tank above the level of the oil pump is even 
worse as to oil leakage. Leakage may then continue throughout a night or 
weekend -- or until the oil has all leaked out. 
These faults are here overcome. By pressurizing with air pump 51 driven 
when the oil pump 41 is driven (by the same drive means 40) the pressure 
is applied promptly upon start-up and ceases to be applied upon shutdown. 
Alternatively, air pump 51 can be normally coupled to the engine which 
drives pump 41, even though the drive for pump 41 may be independently 
decoupled. In other words, the pump is in open communication with and 
supplying pressure to the tank under normal initial operating conditions, 
i.e. before any special operating conditions such as cavitation occur. A 
bleed-vent 56 is provided (unless air leakage is dependable) to dissipate 
the pressure soon after the pump 51 stops. Tank 11 preferably has its 
maximum oil level below the level of pump 41, or its drive shaft, so that 
no oil will leak at this point by gravity. 
If the pump 51 is driven when pump 41 is not, and especially if it may be 
driven in long idle periods, means to discharge the air freely may be 
provided, so as not to maintain pressurization in tank 11. In that event, 
an interlock to ensure closing the discharge when pump 41 is driven would 
be desirable. If valve 53 has an adjustment knob (as some users may 
prefer) and can be set low enough to avoid a positive pressure at the 
drive shaft of pump 41, that may be done. Ideal operation would be to 
pressurize tank 11 just before starting pump 41, to a pressure higher than 
normal, to force the viscous cold oil to the pump 41, and lower the 
setting of valve 53 when the oil has warmed, to be just enough to maintain 
zero pressure at the shaft seal during operation. Some operators may 
prefer to have valve 53 invariable (except for factory or internal 
adjustment) and pump 51 invariably driven with pump 41 for simplicity and 
so that lack of attention can not cause serious consequences. 
If an adjustment knob for valve 53 is provided, a pressure-vacuum gauge 
could be connected adjacent the intake port of pump 41 and the knob could 
be turned to the point that makes the gauge read zero. The zero setting of 
the gauge could be "off" enough to compensate for flow-friction loss into 
the pump, if found desirable. Valve 53 and the gauge could both be located 
in the cab. Making a valve such as 53 responsive to the pressure at the 
intake to pump 41 is another possibility. It would then always supply just 
enough air to tank 11 to yield zero pressure (i.e., the pressure of the 
atmosphere, at the pump intake or shaft). 
Pump 51 should have a capacity only slightly larger than the expected flow 
through vent 56 at 5 lbs., so that the discharge of excess air through 
relief valve 53 will never be objectionable. Indeed, careful correlation 
or an adjustment on vent 56 might be found to make relief valve 53 
unnecessary. 
ACHIEVEMENT 
On start-up, tank 11 is quickly pressurized so that hydraulic pump 41 will 
not draw in air, even with a leaky shaft seal; and so that the shaft seal 
of pump 41 is not subjected to such high pressure differentials as to 
greatly shorten its life. On shutdown, the pressurization will be quickly 
dissipated so that no pool of oil will be found under pump 41 at the next 
start-up time. Because pump 41 never draws in air (or virtually never), 
the oil has the dependable noncompressible characteristics of air-free oil 
that are desired. 
The invention can be added to hydraulic systems already in operation quite 
easily, at least where the hydraulic pump has no disconnect in its drive 
by the engine. Here the added air pump may be battery driven, through the 
ignition or other engine-shut-off switch, so that the air pump will be 
stopped when the engine is turned off. This engine-coupled electric drive 
has an advantage that if the ignition switch is turned "on" a few seconds 
before the engine is started, the tank will be already pressurized when 
the hydraulic pump starts operating.