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Formsprag Clutch • 800-927-3262 Bulletin A-3037
Pre-Installation Instructions
Review Figure 1 to identify the nomenclature for the
various components of the model FB Backstop.
Remove the retainer plate from the stub shaft
adapter by removing the 1/4-20 flat head screws.
Slide the backstop housing off the stub shaft adapter.
Retain the key and screws for later reassembly.
If you have ordered a Backstop with the bore
machined by Formsprag proceed to item 2. If the
backstop was purchased from the factory as a rough
bore, both the C Ring and stub shaft adapter need to
be machined as follows:
1. Bore Machining Instructions
a. C-Ring Assembly. The C-Ring assembly
comes with 0.125” shims installed between
the C-ring halves. Machine the CRing
assembly as a unit with the shims installed
to the maximum size of the mating shaft
diameter with a tolerance of 0.000”
to 0.001”. The surface finish of the
machined C-Ring boreshould be 30 to 45
micro Ra. Skim cut the C-Ring face with the
threaded holes in the same chucking as the
boring operation.
b. Stub shaft. Machine the face opposite the
stub shaft with a blind counter bore to the
maximum size of the mating shaft diameter
with a tolerance +0.001” to +0.002” and a
depth of 0.250”. Machine a 15 degree lead
in chamfer on the counter bore to a depth
not to exceed 0.062”. The run out (TIR)
between the counter borediameter and the
stub shaft diameter must be less than 0.002”.
2. Mating Fan Shaft and Torque Arm Requirements.
Beforeinstalling the backstop, check the
following:
a. Mating Shaft. The mating shaft of the
assembly needs to be round within 0.001”
and machined to a surface finish of less
than 45 micro Ra. The tolerance of the
mating shaft is nominal +0.000” to – 0.001”.
b. Torque arm stops. Ensure that appropriate
equipment frames or structures are available
for fastening stops for the backstop’storque
arm as shown in the Figure 1.
Installation
1. Dry assemble all components to the fan shaft
before completing the assembly with Loctite®to
ensure proper fit.
2. Disassemble the C-Ring assembly into two
halves by removing two 3/8-24 socket head cap
screws. Remove but do not discard the shims.
3. Apply Loctite®262 to the two 3/8-24 socket
head cap screw threads.
4. Reassemble the C-Ring assembly on the fan
shaft with the machined face out. Place the
shims between the C-Ring halves and outside
the dowel and screw holes. The shims will
project out of the C-Ring. Lightly tighten the
socket head cap screws applying just enough
pressure on the shims so they do not fall out
but still can be repositioned.
5. Slide the stub shaft onto the fan shaft. Position
the C Ring assembly by sliding until it mates
against the back face of the stub shaft. Remove
the stub shaft. Tap the CRing toward the shaft
end 1/16 of inch to ensure that the stub shaft
will seat against the C-Ring and not the fan
shaft end. The fan shaft will project 0.200” to
0.250” beyond the C-Ring when positioned
correctly.
6. Without repositioning the C-Ring, remove the
shims and torque down the fasteners a quarter
turn at a time, alternating from one fastener to
the other to tighten the C-Ring evenly. Torque
the 3/8-24 screws to 300 – 450 inch-pounds.
Note: Individually tightening the screws one at
atime to the specified torque will
damage the C-ring and not provide
adequate clamping to the shaft.
7. Slide the stub shaft on the mating shaft. Place
the flat washers on the 5/16-24 screws. Apply
Loctite 262 to the four 5/16 – 24 socket head
cap screw thread and thread them through the
holes in the stub shaft and into the C-Ring,
hand tight.
8. Verify that the stub shaft is concentric with the
fan shaft by keeping the run out on the stub
shaft to within 0.002 in. TIR.
