TM 9-2330-203-14&P
1-9.
DIFFERENCES BETWEEN MODELS (Con't).
b. Internal Wheel Brake Mechanism.
(1) MI 97 and M 198. Air actuated wheel brakes. Cam actuated with one end of brakeshoes anchored.
(2) M1 97A1 and M198A1, Early Models.
Air-over-hydraulic actuated wheel brakes using free-floating
brakeshoe, double-wheel cylinder mechanism.
(3) M1 97A1 and M1 98A1, Late Models. Air-over-hydraulic actuated wheel brakes using single wheel cylinder
at the top, with bottom of brakeshoes anchored. Wheel cylinder is inclined 360 to top rear of axle center.
c. Service Brake System.
(1) M197 and M198. Air system receives air supply from towing vehicle. Intervehicular air hose supplies
semitrailer air system. Airbrake chambers mounted near wheels operate brakes through mechanical linkages (slack
adjusters and airbrake chamber pushrods).
(2) M1 97A1 and Ml 98A1. Air-over-hydraulic system receives air from towing vehicle. Intervehicular air hose
supplies semitrailer air system. Airbrake chamber mounted remotely from wheels operates brakes through hydraulic
linkage (master cylinder and hoses).
e. Wheels.
(1) M197 and Ml 97A1. Dual wheels are mounted on six hub studs. Inner wheel is secured with six cap nuts that
have internal and external threads. Internal threads engage hub stud threads; external threads receive six single ball
seat nuts that secure outer wheel. Stud circle diameter is 8y4 in. (22.2 cm).
(2) Ml198 and M1 98A1. Dual wheels are mounted on ten hub studs and secured with ten cap nuts and ten
single ball seat nuts in the same manner as described above. Stud circle diameter is 11% in. (28.6 cm).
f. Hub Cap.
(1) M1 97 and M1 97A1. Hub cap and gasket are secured to hub with eight hexagon head capscrews and
(2) Ml 98 and M198A1.
Hub cap and gasket are secured to hub with ten hexagon head capscrews and
g. Brakedrums. All brakedrums attach in a similar manner. M197 and M198 use an oil slinger-deflector, M197A1
and M198A1 do not.
h. Frame. Dolly frames are basically of the same structural design. M198 and M198A1 have stronger frames due to
stronger centers reinforced with a crossrail.
i. Spring Brackets. M197 and M198 have welded spring brackets. M197A1 has riveted spring brackets. M198A1
has bolted spring brackets.
j. Lunette. M197and M197A1 have affixed lunette. M198 and M198A1 have a two-position lunette, the upper
position being 6 in. (15.2 cm) higher than the lower position.
k. Springs.
(1) M197 and M198. M197 semi-elliptical springs have ten leaves; M198 semi-elliptical springs have 11 leaves.
Ends of first two leaves are curled at front and suspended from spring mounting bracket with a fluid passage bolt and
bearing. First three leaves are suspended on a spacer and screw in rear brackets. Semi-elliptical helper springs with five
leaves are secured to main spring by the same center bolt that holds main spring leaves together. U-bolts and a mounting
plate attach spring to axle.
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