A drum shaft assembly for a drum dynamometer includes at least one annular inertia element on the shaft and releaseably couplable to the shaft and drum to vary the inertia thereof. The assembly carries first interrupted thread means releasably engageable with second interrupted thread means rotatably supported in the inertia element. A manipulator is slideable axially of the drum and shaft assembly between a stand-by position and a working position where it can offer or receive the inertia element to or from the shaft and drum. The manipulator has a support for supporting the inertia element and a rotatable key operable to bring about relative rotation of the first and second interrupted thread means to fasten the inertia element to or to release the inertia element from the shaft and drum.

FIELD OF THE INVENTION 
The present invention relates to a dynamometer that is of particular 
advantage for the testing of tyres and brakes of aircraft, though it may 
also be used to test other vehicle tyres and brakes, and in general for 
monitoring the acceleration, deceleration or energy tolerance of any 
moving device intended to be coupled to a vehicle. More particularly, the 
invention is concerned with the drum and shaft assembly of such a 
dynamometer. 
BACKGROUND TO THE INVENTION 
The use of a so-called "chassis dynamometer" with inertial weights to 
measure the acceleration and deceleration performance of an automobile or 
other vehicle has long been known (see Knudsen R. F., "Inertia 
Electronically", ISA Journal, April 1958, Vol. 5. No. 4. pp 52-54). 
Further dynamometers are described in Patent Specifications Nos. 
GB-A-1297813, 1604320 and 2149520 and in Roberts J. B., "An Advanced 
High-Speed Dynamometer for Testing Aircraft Tyres, Wheels and Brakes", 
read before the Applied Mechanics Group of the Institution of Mechanical 
Engineers on 10 Apr. 1974. 
This invention is concerned with a dynamometer of the above general kind in 
which a test subject is urged towards a rotatory drum by means of a 
loading head and provision is made for mechanical means to attach or 
detach the or each inertia element from a rotating drum and shaft 
assembly. Thus for tests under high acceleration it may be desirable for 
the drum and shaft assembly to have a low moment of inertia, whereas under 
other test conditions it may be desirable to have a greater drum width and 
inertia. It is understood that the change in drum width and actual inertia 
may be used in addition to inertia simulation as described by Knudsen or 
energy control as described in Patent Specification No. GB-A-2149520. 
Accordingly, the invention provides a drum shaft assembly for a drum 
dynamometer including at least one annular inertia element on the shaft 
and releaseably couplable to the assembly to vary the inertia thereof, 
wherein the assembly carries first interrupted thread means releaseably 
engageable with second interrupted thread means rotatably supported in the 
inertia element, and manipulator means is slideable axially of the drum 
and shaft assembly between a stand-by position and a working position 
where it can offer or receive the inertia element, the manipulator means 
having support means for supporting the inertia element and rotatable key 
means operable to bring about relative rotation of the first and second 
interrupted thread means to fasten the inertia element to or to release 
the inertia element from the assembly. 
Other preferred features of the invention are defined in the appended 
claims to which attention is hereby directed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
In the drawings, a drive shaft assembly 1 comprises a pair of drum-defining 
discs 1a that are shrink-fitted onto a dynamometer shaft 1b that is 
supported for rotation about a generally horizontal axis by means of 
radial and thrust bearings 25, 26 in a bearing housing defined by a pair 
of half-caps 5, inner cover 6 and outer cover 7. A sleeve 1c fits between 
the shaft 1b and the shrunk-fit drum disc 1a. Spacing between the bearings 
25, 26 is maintained by means of a spacer ring 10 and outer and inner 
labyrinth members 8, 9 are interdigitated with the inner and outer covers 
6, 7 to form a seal. The shaft 1b terminates in a coupling 29 for shaft 
drive means and is keyed by drive pins 23 for rotation by the coupling 29. 
To either side of the drum discs 1 there are provided disc-shaped inertia 
elements 2 that may be coupled to the drum and shaft assembly 1 for 
rotation therewith as is the right hand element 2 in FIG. 1 or may be 
retracted therefrom as is the left hand element 2'. The invention provides 
mechanical means by which the inertia elements 2, 2' may be coupled to or 
decoupled from the shaft and drum assembly 1, and for this purpose a bed 
structure 100 has pairs of mountings 18 obliquely under either side of the 
shaft 1b. The mountings 18 provide bearings of parallel mechanisms whose 
other links are support links 11, 12 and ties 22. Bearings 65 in the 
mountings 18 support bottom pins 15 on which the links 11, 12 pivot. The 
upper ends of links 11 and 12 carry support fittings 13 that rotatably 
support top pins 14 having outward extensions that are pivoted in the ties 
22. Inward extensions of the pins 14 are journalled in bushes 16 in 
spherical bearings 66 of side arms 102 of a manipulator frame element 3. A 
cylinder 27 is connected at one end by pivotal mounting 20 to the half-cap 
5 with a rod thereof connected at eye end 28 to rod mounting 21 attached 
to the front frame 3. Accordingly extension of the rod from the fully 
retracted position shown in FIG. 1 advances the frame 3 towards the 
inertia element 2 with the attitude of the frame 3 being maintained 
vertical by means of the parallel mechanisms, the cylinder 27 being 
double-acting to return the frame 3 when required. 
In FIGS. 1, 3 and 4 a male interrupted thread member 31 is bolted by means 
of cap screws 54 to the sleeve 1c, has a male frustoconical support 
surface 31a directed at an angle of about 15 degrees to the axis of the 
shaft 1b at its end adjacent to the disc 1a and teeth 31b defining an 
interrupted thread towards its end remote from the disc 1a. 
The inertia element assembly 2 includes an outer disc 2a that fits onto a 
hub 2b with load-transmitting dowels 2c held in position by inner circlip 
2d. The hub 2b has a female frustoconical surface 2e that when the inertia 
element 2 is in position on the male member 31 mates with the surface 31a 
to support the weight of the inertia element. A retaining ring 34 on the 
outer face of the inertia element 2 assists in holding the disc 2a in 
position on the hub 2b. A female interrupted thread member 32 in the form 
of a ring is rotatably supported on the outer face of the hub 2b with a 
radially projecting flange 32a on its outer face sliding in a groove 
defined between a rebate 105 in the outer face of the hub 2b and the 
retaining ring 34 attached thereto by means of bolts 106. The ring 34 is 
formed at its outer extremity with inturned teeth 34a of an interrupted 
flange. The member 32 is further formed with axially directed castellated 
teeth 32c. The inner face of the ring 32 is formed with inturned teeth 
32b defining a female interrupted thread that in the position shown in 
FIGS. 1, 3 and 4 are engaged behind teeth 31b of the male member 31 to 
prevent the element 2 from becoming displaced axially off the male member 
31. As is apparent in FIG. 5 the angle subtended by each tooth 31b, 32b, 
32c at the axis of the shaft 1b is half (in this instance 15 degrees) of 
the angular extent of the teeth 34a (in this instance 30 degrees). At the 
outer diameter of the element 2 drive pins 45 on the inner face of the 
element locate in sockets in the drum 1. 
In order to be able to remove the inertia element 2 from the drum shaft 
assembly 1, the manipulator frame element 3 carries a rotatable key ring 
33. A flange at the outer end of the ring 3 locates in a rebate in the 
frame element 3 and is held in place by means of a retaining ring 37 that 
is fastened to the outer face of frame element 3 by cap screws 60. The 
outer face of frame element 3 carries a pair of key actuator cylinders 50 
pivoted between the frame element 3 and the key ring 33 which are operable 
to effect locking and release rotational movements of the ring 33. The 
inner end of the ring 33 is inwardly hooked to define interrupted teeth 
33a and is apertured to permit entry of the axially directed castellated 
teeth 32c of the female interrupted thread member of ring 32. Thus on 
rotation of the ring 33 by actuators 50 (a) the teeth 33a are registered 
with or removed from register with the teeth 34a of the retaining ring 34, 
thus attaching the inertia element to or releasing the inertia element 2 
from the manipulator frame element 3, (b) the rotational movement is 
transmitted via the castellated teeth 32c to the female ring 32, and (c) 
the interrupted thread teeth 32b of the female member are registered with 
or removed from register with the teeth 31b of the male interrupted thread 
member 31, thereby fastening the inertia element 2 to or releasing the 
inertia element 2 from the drum shaft assembly 1. The frame element 3 also 
carries on its inner face three spigots 36 that as the element 3 is 
offered to the inertia element 2 enter bushes 35 in the hub 2b which take 
the load of the inertia element 2 when it is on the frame element 3. The 
inertia element 2 is therefore supported at all times either by the 
tapered surface 31a of the male member or by the spigots 36. The periphery 
of the frame element 3 carries inwardly directed set screws 57 that when 
the element 3 is offered to the element 2 butt against the retaining ring 
34 for proper axial positioning of members 2 and 3. 
It will be appreciated that for attachment and removal of the inertia 
elements 2, 2', the drum and shaft assembly 1 and the manipulator element 
3 have to be in a proper angular position. A dedicated DC drive motor is 
therefore provided to inch the drum and shaft assembly round until a datum 
plug 120 (FIG. 2) registers with a sensor 121 (FIG. 2) to indicate that 
the correct angular position has been reached. Thus the plug 120 may be a 
brass insert in the steel of the drum 1a and the sensor 121 may be a 
magnetic proximity sensor that changes state when it is adjacent brass 
rather than steel. 
From the standpoint of safety it is desirable to provide interlocking means 
so that an inertia element 2 or 2' cannot be released from the shaft and 
drum assembly 1 unless the manipulator has been advanced to accept and 
support it. For this purpose a locking pin 42 that is slideable in a bore 
in the hub 2b is biased on the direction of the outer hub face by a coil 
spring 42a or other resilient means to bring its head 42b normally into 
register with the teeth 32a. When the inertia element 2 is on the shaft 
and hub assembly 1 and the manipulator has been retracted, the pin 42 
prevents rotation of the female member or ring 32. An extension 42c on the 
outer face of head 42b passes through the retaining ring 34 to the outer 
face thereof and carries at its extremity a magnetic pad 42d. As the frame 
element 3 is offered to the inertia element 2, it encounters the pad 42d 
and moves the pin 42 inwardly against the resistance of the spring 42a, 
freeing the member 32. An inductive proximity switch 74 detects the pin 42 
and enables the position thereof to be monitored, thereby giving an 
indication whether the inertia element 2 is in a locked or releasable 
state. A further function of the pin 42 is to keep the inertia element 
locked in position on the drum and shaft assembly against the risk that it 
might vibrate loose during a test, and for this purpose it is desirable to 
be able to monitor that locking has in fact occurred. For this purpose, 
when an inertia element 2 or 2' has been fitted to the drum and shaft 
assembly, the manipulator is retracted a small distance, and it is 
ascertained whether the magnetic pad 42d is present protruding the 
expected small distance from the hub of the inertia element. 
It will be appreciated that various modifications may be made to the 
embodiment described above without departing from the invention, the scope 
of which is defined in the appended claims.