  Pneumatic And Hydraulic Equipment Page 428

EXAMPLE: At 80 psig, what is the force using a #22 from 4.0 to 9.0 inches, or 9.0 - 4.0 = 5.0 inch stroke? See [ 8 ] for force at 4.0 inches (7,180 #) and [ 9 ] for force at 9.0 inches (4,670 #). This example illustrates the primary difference between Firestone Airstrokes and conventional air cylinders. Air cylinders have a constant area for the pressure to work against, or constant effective area. The effective area and force of an air spring changes as the height changes . In the example the effective area of a #22, at 4.0 inches using the 80 psi curve, is: 7,180 lbs. 80 Ibs/in 2 = 89.8 in 2 at 9.0 inches in height it is: 4,670 lbs. 80 Ibs/in 2 = 58.4 in 2 An air cylinder with 89.8 in 2 of area would have an 80 psi curve as shown by dotted line [ 10 ] . The volume curve [ 3 ] may also be of importance: a. If one needs to know the amount of free air (then compressed by the compressor) to perform a desired operation. b. If the actuation must be completed quickly and calculations of flow through the air inlet (orifice) are required. In each case above, the change in internal volume is required. Read up from the two heights involved to the intersecting point with the volume curve. Then move to the left and read from the volume scale. In the example at 4.0 a #22 (notice most volume curves are at 100 psig) has an internal volume of 349 in 3 [ 11 ] and at 9.0 the volume is 752 in 3 [ 12 ] . The change in volume is then 752 in 3 - 349 in 3 , or 403 in 3 . The volume at minimum height (349 in 3 ) would not be subtracted if exhausting the air spring to atmospheric pressure. Notice the shaded area [ 13 ] . We do not recommend that an air spring be used at heights extending into this section. The "beginning of the shaded area" for a #22 is at 101 inches [ 5 ] . AIRMOUNT ISOLATION Because of lateral stability considerations we recommend that each air spring be used at a specific height when used as an isolator . This specific height is called the "Airmount design height" [ 6 ] . The vertical line running through this height [ 7 ] is darkened so that it is easy to see where it intersects the static curves for load readings. EXAMPLE: Support a 4,100 pound load with an air spring. Would a #22 be appropriate, and if so, at what height? The height isn't much of a problem, as this part SHOULD BE USED AT 9.5 INCHES. Simply move up the darkened line to where it intersects 4,100 Ibs [ 14 ] . That point falls between the 80 and 60 psig curves. Exactly what pressure would be required? Use the formula: Effective Area = Load (Ibs.) Pressure (Ibs/in 2 ) Determine the effective area at 9.5 inches (using the 80 psig curve, since 80 psig would be closer to our exact pressure than 60 psig), or: Effective Area = 4,280 Ibs.  = 53.5 in 2 80 Ibs/in 2 Then divide the actual load by the effective area: 4,100 Ibs. = 76.6 in 2 53.5 in 2 The pressure required to support 4,100 Ibs. with a #22 at a design height of 9.5 inches is therefore 76.6 PSIG. Please note that the static data can be converted to dynamic data (the air spring is in motion) by applying the formulas that are presented in the Airmount isolation section. INTERNAL RUBBER BUMPERS Some parts are available with internal rubber bumpers. Where a bumper is available, it is shown as a dotted line in the cross sectional view of the air spring. Additionally, please note that: 1. the minimum height is increased to the "bumper contact" point [ 16 ] (this reduces the total available stroke some- what, by 4.2 - 3.0 = 12 inches in our #22 example), and 2. the order block contains the proper ordering numbers for parts with bumpers. ACTUATORS - ISOLATORS Firestone (314) 427-0600 800-444-0522 (FAX) 314-427-3502 www. j hf.com JHF Catalog [ Volume 7 ] 428 Prices Subject to Change Without Notice John Henry Foster pneumatic and hydraulic equipment

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