Hewland NMT Guía del usuario
PAGE 1
© Copyright Hewland Engineering Limited
SERVICING INSTRUCTIONS AND ILLUSTRATED
PARTS LIST FOR HEWLAND NMT GEARBOXES
HEWLAND ENGINEERING LTD
WALTHAM ROAD, WHITE WALTHAM,
Last Update 10th July 2012
MAIDENHEAD, BERKSHIRE,
SL6 3LR, ENGLAND
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© Copyright Hewland Engineering Limited
CONTENTS :-
page page
Technical specification 4 Changing gear ratios 13
General notes 5 Powerflow differential 14
Differential bearing preload 6 Differential Options 15
Pinion shaft mounting 7 Illustrated Parts List 22
Pinion shaft bearing preload 8 Gearbox Tooling 35
Crownwheel backlash setting 9 General Technical Bulletins 36
Sequential barrel setting 10 Oil system layout 38
Gearbox assembly 11 Recommended Tightening 39
Torques
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© Copyright Hewland Engineering Limited
ILLUSTRATIONS :-
page page
fig.1 Differential bearing preload 6 fig.12 Casings & Associated Parts 25
fig.2 Pinion shaft setting 7 fig.13 Layshaft Assembly 27
fig.3 Pinion shaft bearing preload 8 fig.14 Pinion Shaft Assembly 29
fig.4 Crownwheel setting 9 fig.15 Selector Assembly 31
fig.5 Sequential barrel setting 10 fig.16 NLT-218-Tripod Assembly 33
fig.6 Powerflow diff assembly 14 fig.17 Gearbox Tooling 35
fig.7-10 Differential Assemblies 15-21 fig.18 Oil system layout 38
fig.11 Differential Subsidiary Parts 23
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© Copyright Hewland Engineering Limited
TECHNICAL SPECIFICATION
The NMT gearbox is a transaxle unit, designed for mid-engined, rear wheel drive cars. The unit is produced with six
forward gears, reverse, and a powerflow differential.
The gear selection mechanism is sequential, with a separate, mechanically interlocked reverse engagement mechanism.
The drive is taken from the engine via the clutch shaft, which turns input and pinion gears to drive the final transmission
assembly.
Gear changing is effected through non-synchronising face dogs. An extensive range of gear ratios provides a wide
range of gearing requirements. The gear ratios and differential assembly can easily be changed without removing the
gearbox from the vehicle.
Heat treated nickel chrome steel is used to manufacture all gears and shafts. The selector forks are also steel.
Lubrication is by splash or optional internal pump with distribution circuit, and the oil is retained by lipped oil seals.
In general configuration, the NMT-200 is a high tech racing transaxle unit which achieves the maximum effective use of
power, in conjunction with extremely stiff integral rear suspension mountings.
weight (AL) 128 lbs (58 Kg) 1st to 6th gear ratios from 3.31:1 to .89:1
oil type SAE 80 or 90 final drive ratio 11/31, 10/31, 9/31
oil quantity 7 pints (4 litres) clutch shaft made to customer's requirements
max. torque 320 lbs.ft (430 Nm) Pinion shaft nut tightening torque = 120 lbs.ft (163 Nm)
Crownwheelbolts=75lbs.ft(100Nm)
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© Copyright Hewland Engineering Limited
GENERAL NOTES :-
a/ Read these instructions carefully and with reference to the illustrations.
b/ Before dismantling the gearbox, see that a clean tray is available, in which to place the parts.
c/ Thoroughly clean and inspect all parts before reassembly. Discard any worn or damaged components and replace with new
ones.
d/ Use only genuine Hewland parts as replacements. These are manufactured in our workshops to the fine tolerances necessary and
are rigorously inspected.
e/ Always ensure that locknuts, and oil seals are in good condition when reassembling.
f/ All studs and screws must be Loctited or wirelocked in position, unless stated otherwise
g/ Bearing Replacement :-
Bearings can only be removed or renewed if the casings have been warmed in an oven, or with a blowlamp. In the latter case,
keep the blowlamp moving while heating the casing.
Note: Do not overheat. Test with a spot of water which will bounce off at the correct temperature.
Once a casing is heated, all bearings should be pressed into their respective seatings without delay, thus eliminating the need to
reheat. At the correct temperature, fitting the bearings should present no difficulty.
During cooling, or when the casings have cooled, it is advisable to once more lightly press the bearings to ensure that they are
correctly seated.
h/ Oil:
Fill the gearbox through the plug hole on top of the maincase. The oil will find it's own level within the gearbox.
Note: Too much oil will not directly cause any harm, but is undesirable as it may induce power loss and overheating of internals.
PAGE 6
© Copyright Hewland Engineering Limited
DIFFERENTIAL BEARING PRELOAD
Requires special tool No. SK-119-FGC
Assemble the differential case (57) and end cap (58),
and bolt the crownwheel (56) to it. Fit the differential
unit into the maincase (1) using dummy bearings SK-
119-FGC, and fit the sideplate (3). Adjust the shims
(103) if necessary, to achieve 4 – 6 lbs.ft of bearing
preload torque (to turn the diff assembly in it’s bearings
when oiled.).
The table below is to inform you of which sideplate
spacers should be used with which crownwheel when
mounted on a TPT differential. The reason that there are
two sets of spacers, is that the crownwheels have
different mounting distances.
FIGURE 1
PART No. RATIO MOUNTING
DIST
ITEM (90) ITEM (91)
NMT-221-AW 11/31 2.316” FGC-206-2A NMT-205-2
NMT-221-BW 10/31 2.316” FGC-206-2A NMT-205-2
NLT-221-CW 9/35 2.200” FGC-205-2A NMT-205-2A
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© Copyright Hewland Engineering Limited
FIGURE 2
PINION SETTING
Requires special tool No.SK-1470-A & SK-1913
Press the pinion head bearing inner (37) onto the
pinion shaft (4). Fit the bearing housing (37) and
shims (40) into the maincase. Assemble the pinion
shaft into the maincase (1) and fit the bearing carrier
(2). Tighten the pinion shaft nut (111) onto the
pinion shaft until the pinion shaft requires 20-25
lbs.ins to turn it in it's bearings (equivalent to a
tangential force of 16-20 lbs at the outside diameter
of tool SK-1913)
Fit tool SK-1470-A into the maincase diff bearing
bore, and use feeler gauges to measure the gap
between the tool and the pinion front face. This
clearance should comply with the dimension
indicated on the pinion shaft label (also stamped on
the front face of the pinion shaft), and can be
adjusted by adding or removing shims (40) from
behind the pinion head bearing housing (37).
Alternatively, the pinion mounting distance can be
measured with a height gauge, and set to the
dimension on the pinion shaft label.
Note: It is not correct practice to replace a pinion
shaft without measuring the setting distance, even if
the old and new shafts have the same calibration.
SK-1913
PAGE 8
© Copyright Hewland Engineering Limited
PINION BEARING PRELOAD
Requires special tool No.SK-1913
Having installed the correct pinion shaft
bearing shims, remove the bearing carrier
(2). Add the spacer (14) and hubs (20,33) to
the pinion shaft, and replace the bearing
carrier. The thickness of the spacer (14)
should be adjusted, by grinding, to give a
bearing preload torque of 20-25 lbs.ins with
oiled bearings (measure torque as when
setting pinion shaft, using tool SK-1913).
FIGURE 3
It is essential that the pinion bearing preload
is checked and adjusted if any of the pinion
shaft components are replaced, with the ex-
ception of the locknut and locking ring.
Fitting a longer six gear spacer (14) will de-
crease the pinion bearing
preload, whereas shortening the six gear
spacer (14) will increase the pinion bearing
preload.
SK-1913
PAGE 9
© Copyright Hewland Engineering Limited
CROWNWHEEL & PINION BACKLASH SETTING
Requires special tool No. SK-1913
With the pinion shaft correctly fitted, and the correct diff
bearing shims ascertained, the actual backlash can be
measured by means of a dial test indicator against the
notch in the outside diameter of tool SK-1913. Be sure to
take at least 6 backlash readings, turning the crownwheel
30-45 degrees between each reading (this is to ensure
that any variation due to manufacturing tolerances are
taken account of.
The correct backlash figure should be taken from the
card supplied with the gears. If the measured backlash is
incorrect, rectify it by removing some shims (103) from
behind one diff bearing, and inserting them behind the
other, thus moving the diff across in the maincase. Do
not add or discard any shims at this stage, as to do so
would affect the diff bearing preload.
Once the correct backlash has been achieved, replace the
dummy bearings with bearings (83) and confirm that the
backlash is correct.
Note: Dummy are used so that it is easy to change the shims
during the setting procedures. Before fitting the actual diff
bearings, it is important compare thier width with that of the
dummy bearings and compensate the shims accordingly for any
difference.
FIGURE 4
PAGE 10
© Copyright Hewland Engineering Limited
FIGURE 5
SEQUENTIAL BARREL SETTING - Requires special tool No.SK-1451
a/ Assemble the barrel (15), spacer (26), bearing retaining plate (25),
bearing (27), and nut (29). Heat the bearing carrier (2), slide the barrel
assembly into the bearing carrier, and secure using screws (12).
b/ Place the roller (24) onto the detent plunger (21), and insert them into
the bearing carrier (2). Add the detent spring (22), bonded seal (115),
and detent plug (23).
c/ Slide the selector forks (17) over the barrel (15), and secure using
selector pins (18). For final assembly, Loctite the pins (18) into the forks
(17).
d/ Stack the pinion gears (16), hubs (19,20), clutch rings (19) and spacer
(14) in place in the bearing carrier, and slide the whole thing onto the
dummy shaft of fixture SK-1451.
e/ Tighten the pinion shaft nut (111) onto the end of the dummy shaft,
ensuring that the disc spring on the fixture preloads the bearings.
f/ Rotate the barrel to engage first gear. Measure and record the gap
between the dogfaces of third through sixth gears. Engage third gear and
repeat the measurement for first and second dogfaces. It is important
that these dimensions are not taken when the barrel is in the neutral
position.
First, third and fifth gear dog gap measurements will be similar (as will
second, fourth and sixth). Any difference between the odd & even gear
measurements must be corrected by replacing the barrel spacer (26) with
one of the correct thickness.
Note: It is not possible (or necessary) to individually adjust each fork.
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