When one considers the large number and the regularity of quotation requests and inquiries that Sheffer receives, it can be seen that there are many users of hydraulic cylinders that have applications requiring stainless steel construction. Typically, these requests and inquiries address N.F.P.A. hydraulic cylinders with stainless steel construction and (in referencing the N.F.P.A.) it appears that some users are under the assumption that stainless steel hydraulic cylinders will have the same pressure ratings and dimensional interchangeability as Sheffer’s conventional product lines, in particular “HH” and “MH” Series.
The 3 Key Aspects to Know
When specifying N.F.P.A. hydraulic cylinders with stainless steel construction, three aspects can be compromised that many cylinder users fail to recognize, or of which they are unaware:
1. Pressure ratings have to be reduced
2. N.F.P.A. dimensional interchangeability can be affected
3. Design Factor Of Safety (F.S.) is altered
Our standard product lines, in accordance with N.F.P.A., have recommended operating pressures as depicted in our product literature. These operating pressures are based on a F.S., usually 3: 1, and the material yield strength of the components compromising the pressure containing part of the cylinder, namely the tube, tie rods, piston to rod connection, and the gland retaining screws. In standard construction, the cylinder components are manufactured from carbon steels with yield strengths ranging from 70 K.S.I. for the tube to 153 K.S.I. for the gland screws.
The grades of stainless steel available for cylinder construction have approximately one half the material yield strength of carbon steel, 30 – 35K.S.I. Therefore, if N.F.P.A. interchangeability is to be maintained, either the operating pressure for the cylinder has to be reduced accordingly or the F.S. has to be compromised. If the operating pressure is not reduced, then the dimensions of the pressure containing components have to be altered, e.g. thicker tube wall, larger tie rod diameter, oversize piston to rod connection, and the number or size of the gland screws.
In considering the above concepts, as a standard procedure, we perform the necessary calculations for the pressure containing cylinder components for every quotation request or inquiry requiring stainless steel construction. A proper pressure rating and/or dimensional interchangeability is determined and this allows us to furnish a quality cylinder with an acceptable F.S. and the expected structural integrity.
Are there any other criteria you find fundamental to know when specifying N.F.P.A. hydraulic cylinders with stainless steel construction? Leave a comment with your thoughts below.
Image credit: Stuart Miles