The chart on this page shows the fluid pressure rating of mild steel tubing  used for cylinder barrels. Tubing inside diameter (bore) is shown in the first column, and wall thickness along the top of the chart.

The chart includes an ample safety factor to take care of variations in composition or wall thickness of production tubing, pressure spikes generated in the hydraulic system, and metal fatigue caused by high cycle rates. More complete information is given on the opposite side of this sheet.

Safe Working Pressure
The calculation of safe working pressure on steel tubing used for construction of hydraulic cylinders is not an exact science because there are possible variations in the tubing material plus many other variables such as these:

1. SAE grade and hardness of he tubing steel.
2. SAE grade and hardness of he tubing steel.
3. Whether the tubing steel is ductile or brittle.
4. Whether the cylinder will be constructed with "floating" ends or with confined (welded) ends.
5. Ratio of wall thickness to inside diameter of tubing.
6. Operating temperature (if extremely high).
7. Cycle rate. (High cycle rate tends to fatigue tubing).

Formula for Calculating Pressure Rating
Several formula may be used to calculate wall thickness for a desired hydraulic working pressure, but two of them seem to be used more than the others for ductile steel tubing. Lame's formula is used for thick wall tubing. This includes tubing with a wall thickness greater than 10% of its inside diameter. If the wall thickness is less than 10% of the I.D., the tubing is considered as "thin wall", and Barlow's formula gives more accurate results. Most of the tubing used to plumb a hydraulic system is "thin wall", and its pressure rating can be calculated with the same formula given here for cylinder barrel thin wall tubing.

Material for Cylinder Barrels
The most common material for hydraulic cylinder barrels seems to be low carbon steel, such as SAE 1020, finish annealed, or plain low carbon cold drawn seamless steel tubing, with a hardness of about 84 Rockwell B, and having a tensile yield point of 60,000 PSI (mechanical). This material has been used to build cylinders rated at 6,000 PSI working and up to 5-inch bore. To build cylinders with higher pressure rating or larger bore, a ductile steel with higher yield point should be used. Cast iron (a brittle material) should never be used for pressure ratings over 2,000 PSI regardless of wall thickness.

Working Pressure of Steel Tubing for Cylinder Barrels

 Bore (Inside Diameter Wall Thick., 1/8˝ 0.125 Wall Thick., 3/16˝ 0.1875 Wall Thick., 1/4˝ 0.250 Wall Thick., 3/8˝ 0.375 Wall Thick., 1/2˝ 0.500 Wall Thick., 5/8˝ 0.625 Wall Thick., 3/4˝ 0.750 Wall Thick., 7/8˝ 0.875 Wall Thick., 1˝ 1.000 Wall Thick., 1-1/4˝ 1.250 Wall Thick., 1-1/2˝ 1.500 Wall Thick., 1-3/4˝ 1.750 1.50 1600 2195 2800 3845 4705 - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1.75 1380 2045 2460 3425 4235 - - - - - - - - - - - - - - - - - - - - - - - - - - - - 2.00 1250 1790 2350 3080 3845 4505 - - - - - - - - - - - - - - - - - - - - - - - - 2.25 1110 1510 2100 2800 3520 4150 - - - - - - - - - - - - - - - - - - - - - - - - 2.50 1000 1445 1905 2565 3245 3935 4380 - - - - - - - - - - - - - - - - - - - - 2.75 910 1365 1740 2545 3005 3580 4095 - - - - - - - - - - - - - - - - - - - - 3.00 835 1250 1600 2350 2800 3350 3845 4295 - - - - - - - - - - - - - - - - 3.25 770 1155 1480 2175 2620 3110 3620 4060 - - - - - - - - - - - - - - - - 3.50 715 1070 1380 2035 2460 2935 3435 3845 4235 - - - - - - - - - - - - 4.00 625 940 1250 1790 2350 2655 3080 3480 3845 - - - - - - - - - - - - 4.50 555 835 1110 1600 2100 2405 2800 3170 3520 4150 - - - - - - - - 5.00 500 750 1000 1445 1905 2195 2565 2915 3245 3845 - - - - - - - - 5.50 455 680 910 1365 1740 2495 2365 2695 3005 3580 4100 - - - - 6.00 415 625 835 1250 1600 1980 2195 2505 2800 3350 3845 - - - - 6.50 385 575 770 1155 1480 1835 2175 2340 2620 3145 3625 4060 7.00 355 535 715 1070 1380 1710 2030 2195 2460 2960 3425 3845 7.50 335 500 665 1000 1335 1600 1905 2065 2320 2800 3245 3655 8.00 310 470 625 935 1250 1500 1790 2070 2195 2655 3080 3480 8.50 295 440 590 880 1175 1420 1690 1955 2080 2525 2935 3320 9.00 275 415 555 839 1110 1345 1600 1855 2100 2405 2800 3170 9.50 265 395 525 790 1050 1395 1520 1760 1995 2295 2675 3040 10.0 250 375 500 750 1000 1250 1445 1675 1905 2195 2565 2915 10.5 240 355 475 715 950 1190 1380 1600 1835 2245 2460 2800 11.0 225 340 455 680 910 1135 1365 1415 1740 2145 2365 2695 11.5 215 325 435 650 870 1085 1305 1465 1665 2060 2275 2595 12.0 210 310 415 625 835 1040 1250 1410 1600 1980 2195 2505 12.5 200 300 400 600 800 1000 1200 1355 1540 1905 2120 2420 13.0 190 290 385 575 770 960 1155 1345 1480 1835 2175 2340 13.5 185 275 370 555 740 925 1110 1295 1430 1770 2145 2265 14.0 180 270 355 535 715 895 1070 1250 1380 1710 2035 2195 Pressures to the left of the shaded area were calculated by Barlow's formula. Those to the right of shaded area by Lame's formula. Pressures in shaded area are average values from calculations by both formula.

Explanation of Chart
Working Pressure: The chart is based on a mechanical stress, S, of 10,000 PSI in the tubing material. This is fairly conservative, and gives a safety factor of 5:1 for steel tubing rated for 50,000 PSI yield, or a safety factor of 6:1 for steel tubing rated for 60,000 PSI yield strength. While it may, on some applications, be permissible to use a lower safety factor, under no circumstances would we recommend a safety factor less than 2.5 because of pressure spikes in the hydraulic system, and variations in composition or wall thickness of the tubing. Using a safety factor of 2.5, tubing of dimensions shown in the chart could be used at pressures about twice those shown.

Thin Wall Tubing. Fluid pressure values to the left of the shaded area were calculated by Barlow's formula as explained in the box on this page. These are for tubing sizes where wall thickness is less than 7% of the tubing I.D.
Thick Wall Tubing. Fluid pressure values to the right of the shaded area were calculated by Lame's formula:

PSI = S × (OR² − IR² ) + (OR² + IR² ), where:

PSI is fluid pressure;
S is rated yield strength of the material, in PSI;
OR is the outside radius, in inches;
IR is the inside radius, in inches.

This area of the chart includes tubing with wall thickness greater than 10% of the tubing I.D. All areas of the chart are based on the design stress noted above, 10,000 PSI (mechanical), also including the safety factors noted above.

Intermediate Wall Thickness. The shaded area of the chart on the opposite side of this sheet includes tubing with wall thickness between 7% and 10% of tubing I.D. In an attempt to obtain more realistic results, pressure values were calculated with both the thin wall and thick wall formula and the two results were averaged.

Tubing Sizes Not Shown in Chart
The formula and general rules given above may be used to determine working pressure of other tubing sizes, or tubing of other material which is ductile, if its tensile yield strength is known.

© 1990 by Womack Machine Supply Co. This company assumes no liability for errors in data nor in safe and/or satisfactory operation of equipment designed from this information.