Why Filter Your Fluid

Some experts say that up to 90% of hydraulic component failures are caused by contamination. Even if the 90% figure is high, there's no doubt the vast majority of hydraulic component failures are caused by contamination.

When you filter your hydraulic fluid properly, you extend the life of your hydraulic components, help keep them covered under the manufacturer's warranty, and reduce your downtime - all very good reasons to filter your hydraulic fluid.

How Big Is That?

Before we talk about the different kinds of filters, it might be helpful to get an idea of the size of the particles you want to remove from your fluid power system. Most manufacturers of "standard" industrial and mobile hydraulic components require a minimum of 10 micron filtration to keep their warranty in effect. When it comes to keeping your hydraulic system running, 8 micron filtration would be better than 10 micron filtration, and 3 micron filtration would better than 8 micron filtration.

When we're talking about 10 micron and 8 micron and even 3 micron, most folks have absolutely no idea how small those particles are. Hopefully this will help put things into perspective:

40 Microns is the lower limit of visibility with the naked eye. If you can see it, then it is 40 micron or larger.

25 Microns is the average size of a white blood cell. If you cut yourself and bleed into your hydraulic fluid, your white blood cells would probably void the manufacturer's warranty on your pump and other components.

8 Microns is the average size of a red blood cell - we're talking some pretty small stuff here, but if you have a servo valve in your system, this is how fine your filtration needs to be.

2 Microns is the average size of bacteria.

As you can see, we're talking about some pretty small particles. And the better job you do of keeping those small particles from your hydraulic components, the longer your components will last.

Types of Filters

Suction Strainer - A suction strainer is typically installed in the tank or reservoir. Most suction strainers are 100 mesh (149 micron), and are designed to keep really big contaminant particles from reaching your pump. The big concern with suction strainers is you run the risk of cavitating your pump if they get clogged. Suction strainers don't do a real good job of filtering your fluid.

Pressure Filter - A pressure filter is typically placed inline, just after your hydraulic pump or just before an especially sensitive component (such as a servo valve). Because of the relatively high initial cost of the filter and the relatively high price of replacement elements, pressure filters are typically used on systems with servo valves. Pressure filters are typically rated from 3 to 8 micron and have polyester or wire mesh media. The effectiveness of pressure filters is expressed as a beta ratio.

Return Line Filter - The return line filter goes downstream of the components in your system. It is typically mounted on your reservoir and filters your hydraulic fluid just before the fluid returns to the reservoir. Return line filters are typically rated for 10 micron (sometimes 25 micron), and have media made of paper or polyester.

Kidney Loop Filter - This really isn't a kind of filter, but rather a filtering system where you use an auxiliary pump to draw fluid from one portion of your reservoir, pump it through a filter, and then return the fluid to tank.

Filter Buggy - Another filtering system. A filter buggy is typically connected to a hydraulic reservoir on a regularly scheduled basis to provide supplemental filtration. Many companies install downlines with quick disconnect couplers on their reservoirs, allowing the filter buggy to be easily hooked up to the reservoir.

Hydraulic Reservoir - A well designed hydraulic reservoir will help filter your hydraulic fluid. The reservoir will have a baffle and the suction port will be on one side of the baffle while the return line is on the other side of the baffle - forcing the hydraulic fluid to travel all the way around the outside of the reservoir as it travels from the return line to the suction line. As the fluid makes the long trip around the reservoir, contamination drops out of the fluid and accumulates on the bottom of the reservoir.

Filtered Breather Cap - okay, you're right - a filtered breather cap is not a hydraulic fluid filter. However, every time a cylinder in your system extends, the fluid level in your reservoir drops and air is drawn into the reservoir. The filtered breather cap keeps any particles larger than 40 micron out of your reservoir (and 10 micron filtered breather caps are available). As with the suction strainer, you run the risk of cavitating your pump if the filtered breather cap gets clogged.

Some Notes About Your Hydraulic Fluid

The hydraulic fluid in your system needs to transmit power effectively, lubricate the hydraulic components in your system, and transmit heat away from your hydraulic components. To get the best the performance of your hydraulic fluid, it has additives which inhibit corrosion and oxidation, reduce wear, and reduce foaming. Because of the special characteristics of a good hydraulic fluid, you should never put any fluid in your hydraulic system except clean hydraulic fluid.

How clean is your hydraulic fluid? When was the last time you took a fluid sample and had it evaluated? The folks who manufactured your car probably recommend that you change your oil and filter every 3,000 miles. Let's say you stretch it and change the oil and filter every 5,000 miles. If you drive an average of 45 miles an hour, that works out to new oil and filter every 112 hours of operation. If you changed the filter and fluid in your hydraulic system on the same schedule, and your equipment operated just six hours a day, five days a week, you'd be changing your hydraulic filter and fluid every four weeks. No, we're not suggesting that you change your hydraulic filter and fluid every four weeks. We are, however, suggesting that you take fluid samples and have them evaluated on a regular basis so you know for sure how often you should be changing your filters and how often you should be replacing your hydraulic fluid.

And, speaking of replacing your hydraulic fluid, how clean is the new fluid you put in your reservoir? Many fluid suppliers do not do a real good job of filtering the fluid before they send it, and there's a good chance the drum containing your fluid wasn't cleaned real well before they put your fluid in it. It is a very good idea to filter your fluids before adding them to your hydraulic system.

Summary

To summarize the key points:

- You should have adequate filters on your hydraulic system,

- The particles you need to filter out are smaller than white blood cells,

- Your reservoir should have a filtered breather cap,

- You should have your fluid evaluated on a regular basis,

- You should replace your filter elements and hydraulic fluid as needed (based on the results of your fluid evaluation),

- Never put anything but clean hydraulic fluid in your system,

- Filter your fluid before it goes into your reservoir,

- If you cut yourself, don't bleed into your reservoir - your white blood cells may void your warranty.

Fluid Power Formulas

The following formulas are readily available in many engineering textbooks, fluid power design guides, and hydraulic handbooks.

Every effort has been made to insure the accuracy of the formulas and the examples shown. However, it is possible that a typographical error or two has slipped in. Please use some common sense, double check any results that don't seem right, and consult an engineer or certified fluid power specialist for any critical applications.

PUMP CALCULATIONS

Horsepower Required to Drive Pump:
GPM X PSI X .0007 (this is a 'rule-of-thumb' calculation)

How many horsepower are needed to drive a 10 gpm pump at 1750 psi?

GPM = 10
PSI = 1750

GPM X PSI X .0007 = 10 X 1750 X .0007 = 12.25 horsepower

Pump Output Flow (in Gallons Per Minute):
RPM X Pump Displacement / 231

How much oil will be produced by a 2.21 cubic inch pump operating at 1120 rpm?

RPM = 1120
Pump Displacement = 2.21 cubic inches
RPM X Pump Displacement / 231 = 1120 X 2.21 / 231 = 10.72 gpm

Pump Displacement Needed for GPM of Output Flow:
231 X GPM / RPM

What displacement is needed to produce 7 gpm at 1740 rpm?

GPM = 7
RPM = 1740
231 X GPM / RPM = 231 X 7 / 1740 = 0.93 cubic inches per revolution

CYLINDER CALCULATIONS

Cylinder Blind End Area (in square inches):
PI X (Cylinder Radius) ^2

What is the area of a 6" diameter cylinder?

Diameter = 6"
Radius is 1/2 of diameter = 3"
Radius ^2 = 3" X 3" = 9"
PI X (Cylinder Radius )^2 = 3.14 X (3)^2 = 3.14 X 9 = 28.26 square inches

Cylinder Rod End Area (in square inches):
Blind End Area - Rod Area

What is the rod end area of a 6" diameter cylinder which has a 3" diameter rod?

Cylinder Blind End Area = 28.26 square inches
Rod Diameter = 3"
Radius is 1/2 of rod diameter = 1.5"
Radius ^2 = 1.5" X 1.5" = 2.25"
PI X Radius ^2 = 3.14 X 2.25 = 7.07 square inches
Blind End Area - Rod Area = 28.26 - 7.07 = 21.19 square inches

Cylinder Output Force (in Pounds):
Pressure (in PSI) X Cylinder Area

What is the push force of a 6" diameter cylinder operating at 2,500 PSI?

Cylinder Blind End Area = 28.26 square inches
Pressure = 2,500 psi
Pressure X Cylinder Area = 2,500 X 28.26 = 70,650 pounds

What is the pull force of a 6" diameter cylinder with a 3" diameter rod operating at 2,500 PSI?

Cylinder Rod End Area = 21.19 square inches
Pressure = 2,500 psi
Pressure X Cylinder Area = 2,500 X 21.19 = 52,975 pounds

Fluid Pressure in PSI Required to Lift Load(in PSI):
Pounds of Force Needed / Cylinder Area

What pressure is needed to develop 50,000 pounds of push force from a 6" diameter cylinder?

Pounds of Force = 50,000 pounds
Cylinder Blind End Area = 28.26 square inches
Pounds of Force Needed / Cylinder Area = 50,000 / 28.26 = 1,769.29 PSI

What pressure is needed to develop 50,000 pounds of pull force from a 6" diameter cylinder which has a 3" diameter rod?

Pounds of Force = 50,000 pounds
Cylinder Rod End Area = 21.19 square inches
Pounds of Force Needed / Cylinder Area = 50,000 / 21.19 = 2,359.60 PSI

Cylinder Speed (in inches per second):
(231 X GPM) / (60 X Net Cylinder Area)

How fast will a 6" diameter cylinder with a 3" diameter rod extend with 15 gpm input?

GPM = 6
Net Cylinder Area = 28.26 square inches
(231 X GPM) / (60 X Net Cylinder Area) = (231 X 15) / (60 x 28.26) = 2.04 inches per second

How fast will it retract?

Net Cylinder Area = 21.19 square inches
(231 X GPM) / (60 X Net Cylinder Area) = (231 X 15) / (60 x 21.19) = 2.73 inches per second

GPM of Flow Needed for Cylinder Speed:
Cylinder Area X Stroke Length in Inches / 231 X 60 / Time in seconds for one stroke

How many GPM are needed to extend a 6" diameter cylinder 8 inches in 10 seconds?

Cylinder Area = 28.26 square inches
Stroke Length = 8 inches
Time for 1 stroke = 10 seconds
Area X Length / 231 X 60 / Time = 28.26 X 8 / 231 X 60 / 10 = 5.88 gpm

If the cylinder has a 3" diameter rod, how many gpm is needed to retract 8 inches in 10 seconds?

Cylinder Area = 21.19 square inches
Stroke Length = 8 inches
Time for 1 stroke = 10 seconds
Area X Length / 231 X 60 / Time = 21.19 X 8 / 231 X 60 / 10 = 4.40 gpm

Cylinder Blind End Output (GPM):
Blind End Area / Rod End Area X GPM In

How many GPM come out the blind end of a 6" diameter cylinder with a 3" diameter rod when there is 15 gallons per minute put in the rod end?

Cylinder Blind End Area =28.26 square inches
Cylinder Rod End Area = 21.19 square inches
GPM Input = 15 gpm
Blind End Area / Rod End Area X GPM In = 28.26 / 21.19 * 15 = 20 gpm

MOTOR CALCULATIONS

GPM of Flow Needed for Fluid Motor Speed:
Motor Displacement X Motor RPM / 231

How many GPM are needed to drive a 2.51 cubic inch motor at 1200 rpm?

Motor Displacement = 2.51 cubic inches per revolution
Motor RPM = 1200
Motor Displacement X Motor RPM / 231 = 2.51 X 1200 / 231 = 13.04 gpm

Fluid Motor Speed from GPM Input:
231 X GPM / Fluid Motor Displacement

How fast will a 0.95 cubic inch motor turn with 8 gpm input?

GPM = 8
Motor Displacement = 0.95 cubic inches per revolution
231 X GPM / Fluid Motor Displacement = 231 X 8 / 0.95 = 1,945 rpm

Fluid Motor Torque from Pressure and Displacement:
  PSI X Motor Displacement / (2 X PI)

How much torque does a 2.25 cubic inch motor develop at 2,200 psi?

Pressure = 2,200 psi
Displacement = 2.25 cubic inches per revolution
PSI X Motor Displacement / (2 x PI) = 2,200 X 2.25 / 6.28 = 788.22 inch pounds

Fluid Motor Torque from Horsepower and RPM:
Horsepower X 63025 / RPM

How much torque is developed by a motor at 15 horsepower and 1500 rpm?

Horsepower = 15
RPM = 1500
Horsepower X 63025 / RPM = 15 X 63025 / 1500 = 630.25 inch pounds

Fluid Motor Torque from GPM, PSI and RPM:
GPM X PSI X 36.77 / RPM

How much torque does a motor develop at 1,250 psi, 1750 rpm, with 9 gpm input?

GPM = 9
PSI = 1,250
RPM = 1750
GPM X PSI X 36.7 / RPM = 9 X 1,250 X 36.7 / 1750 = 235.93 inch pounds

FLUID & PIPING CALCULATIONS

Velocity of Fluid through Piping
0.3208 X GPM / Internal Area

What is the velocity of 10 gpm going through a 1/2" diameter schedule 40 pipe?

GPM = 10
Internal Area = .304 (see note below)
0.3208 X GPM / Internal Area = .3208 X 10 X .304 = 10.55 feet per second

Note: The outside diameter of pipe remains the same regardless of the thickness of the pipe. A heavy duty pipe has a thicker wall than a standard duty pipe, so the internal diameter of the heavy duty pipe is smaller than the internal diameter of a standard duty pipe. The wall thickness and internal diameter of pipes can be found on readily available charts.

Hydraulic steel tubing also maintains the same outside diameter regardless of wall thickness.

Hose sizes indicate the inside diameter of the plumbing. A 1/2" diameter hose has an internal diameter of 0.50 inches, regardless of the hose pressure rating.

Suggested Piping Sizes:

• Pump suction lines should be sized so the fluid velocity is between 2 and 4 feet per second.
• Fluid return lines should be sized so the fluid velocity is between 10 and 15 feet per second.
• Medium pressure supply lines should be sized so the fluid velocity is between 15 and 20 feet per second.
• High pressure supply lines should be sized so the fluid velocity is below 30 feet per second.

HEAT CALCULATIONS

Heat Dissipation Capacity of Steel Reservoirs:
0.001 X Surface Area X Difference between oil and air temperature

If the oil temperature is 140 degrees, and the air temperature is 75 degrees, how much heat will a reservoir with 20 square feet of surface area dissipate?

Surface Area = 20 square feet
Temperature Difference = 140 degrees - 75 degrees = 65 degrees
0.001 X Surface Area X Temperature Difference = 0.001 X 20 X 65 = 1.3 horsepower

Note: 1 HP = 2,544 BTU per Hour

Heating Hydraulic Fluid:
1 watt will raise the temperature of 1 gallon by 1 degree F per hour

and

Horsepower X 745.7 = watts

and

Watts / 1000 = kilowatts