The Class 1B design utilizes a rectangular shape bead around the entire circumference of its mounting edge allowing the flange bead to be clamped inside the cylinder bore. This design offers a minimum outside housing diameter which is slightly larger than the diaphragm cylinder bore.

In some cases, this design eliminates the need for flange bolts or flange studs. It requires no bolt holes or perforations through the flange of the diaphragm.

Inversion of the piston corner radius is required to form the convolution. In order to retain the piston corner radius and prevent it from returning to its “as molded” shape, a curved lip retainer is recommended.

Because the Class 1B design provides for clamping the flange bead inside of the cylinder bore, the stroke capability is normally limited to the half-stroke in the SB direction only.

The cross-sectional area of the groove for retaining the bead should be equal to the cross-sectional area of the Bellofram bead itself. The corners of the groove should be machined square with minimum fillets. These practices are recommended to provide best retention of the bead. Elastomers are not compressible, and if the groove area is too small, it will definitely damage the rolling diaphragm.

It is recommended this design be used where differential pressures do not exceed 150 psi.

DC 1.00 to 2.50 2.51 to 4.00 DC 4.01 to 8.00 8.01 and up
H As required to yield design stroke (See standard size tables and Note 2 below.)
DC Tolerances on DC and DP are ± .010" per inch of diameter but the tolerance will be no less than ±.010" or greater than ±.060"
WH .020 ± .005 .030 ± .005 .035 ± .005 .045 ± .007
WSW .017 ± .003 (Code “C”) .024 ± .004 (Code “D”) .035 ± .005 (Code “F”) .045 ± .007
(Code “H”)
A .025 Max. .035 Max. .040 Max. .056 Max.
B .025 Max. .035 Max. .040 Max. .056 Max.
E 1/16 R. 3/32 R. 1/8 R. 1/8 R.
F 1/32 R. 3/64 R. 1/16 R. 1/16 R.
WB .080 ± .003 .100 ± .003 See note 3 .120 ± .003
See note 3
.160 ± .003
See note 3
HB .150 ± .005 .200 ± .005 .260 ± .005 .300 ± .005
WB Dia. .121 ± .005 .151 ± .005 .242 ± .010 .242 ± .010

1. This radius is not the piston radius since the head corner will be inverted at assembly
2. Height should not exceed the bore (DC). Tolerance on height to be no less than ±.015" per inch of height.
3. This tolerance does not include sidewall variation
4. Trim tolerances
Hole Diameter OD Trim
Diameter Tolerances
0-1.00" ±.010"
1.01-3.00" ±.015"
over 3.01" ±.020"
5. This “V” rib is for diaphragm processing only. It may not appear on all diaphragms. It is not functional and need not be completely filled. Rib is normally on rubber side of diaphragm.
6. Number, size, spacing and location of “V” ribs may be modified to suit specific beads, or may be left off altogether.
7. Dimensions and tolerances pertain to Bellofram Rolling Diaphragms as manufactured and not to dimensions and tolerances of mating parts.