|1.||A sprocket drive for a chain or tracked vehicle in which the sprocket shaft is connected to the drive shaft of the motor through a gear linkage in which the gear teeth are shaped so as to provide a rate of change of the pressure angle so that the angular velocity ratio of the gear linkage and hence the tangential velocity component of the sprocket is varied cyclically in a fixed phase relationship with the sprocket tooth/chain link engagement sequence.|
|2.||A drive mechanism for endless linked chain or track comprising a sprocket wheel, the shaft of which is driven via a gear linkage and said sprocket wheel drivingly en¬ gaging said links wherein the gear teeth of the gear linkage are shaped so as to provide a rate of change of the pressure angle so that the angular velocity ratio of the gear linkage and hence the tangential velocity component of the sprocket is varied cyclically in a fixed phase relationship with the sprocket tooth/chain link engagement sequence.|
This invention relates to tracked vehicles particularly tracked combat vehicles.
Conventionally the flexible track which supports and propels such vehicles is, particularly in the case of heavier vehicles, made of rigid shoes joined by pins forming a hinge, into a continuous loop.
The track is driven by a sprocket which commonly has 10 to 15 teeth. Conventionally, in meshing, one sprocket tooth engages with a single mating feature on each shoe or pin.
The pitch of the track and hence the circum¬ ferential pitch of the sprocket is commonly in the range of 100 to 175 mm. In operation the track approaches the sprocket at a sensibly constant angle relative to a fixed reference frame attached to the vehicle.
This operation can best be understood with reference to Figure 1 of the drawings. The meshing cycle ■ is considered from the time at which tooth 1 comes into contact with shoe mating point, a, until tooth 2 contacts mating point, b.
In a conventionally driven sprocket the velocity of the common contact (1, a) is essentially constant circumferentially J being° eq^ual to r m where m is the constant angular velocity of the sprocket whose radius is r.
The component of this velocity along the approach line of the track varies as the sprocket turns, being rW m Cos (- A/2) at the moment of first contact, r m when the line 0,a is normal to approach line of the track and r m Cos (A/2) at the end of the cycle when tooth 2 contacts point b.
The common contact point also goes through a cycle of velocity change normal to the approach line of the track, from rWm Sin (-A/2) throug σ h zero to rWm sin
(A/2) during the same time interval.
Velocity changes along the line of the track are resisted by the inertia of the track itself and the inertia of the vehicle which is being driven by the track. In consequence fluctuating torques are produced in the drive line driving the sprocket. Tension fluctuation in the track and vibrating forces, which are input into the vehicle via the sprocket and in some vehicle designs, the idler wheel, are also produced.
These vibrating forces particularly in tracked vehicles driven at high speed produce vibrations which have harmful effects not only on the maintenance and condition of the vehicle itself but also on its occupants .
Prior attempts to ameliorate this problem have concentrated on providing vibration damping by utilizing resilient mountings for elements of the sprocket wheel. Australian Patent 460782 provides a soleplate which incorporates a resilient pad to reduce vibration stress .
Australian Patent 443612 discloses a sprocket construction to distribute driving forces around the periphery of the sprocket to reduce wear. This patent does not address the basic kinematic difficulty of the sprocket drive.
U.S.A. Patent 3486574 discloses a sprocket wheel drive which utilizes 3 sets of meshing teeth on the sprocket and pinions to provide a positive smooth drive and avoids vibrations caused by foreign matter accumulat¬ ion between the sprocket teeth and pinion.
It is an object of this invention to reduce the vibrations emanating from the sprocket drive.
To this end the present invention provides a sprocket drive for a chain or tracked vehicle in which the sprocket shaft is connected to the drive shaft of the motor through a gear linkage in which the gear teeth are shaped so as to provide a rate of change of the
pressure angle so that the angular velocity ratio of the gear linkage and hence the tangential velocity component of the sprocket is varied cyclically in a fixed phase relationship with the sprocket tooth/chain link engagement sequence.
The result of this modification to the gear teeth is a reduction in the degree of torque fluctuations transmitted to the chain or vehicle track and the drive line and driving motor thereof and consequently a reduction in the level of vibration.
The analysis of a simplified track/sprocket kinematic model gives a good indication of the sort of angular velocity variation required and one which can usefully be used in practice. In the drawings:
Figure 1 is a schematic representation of a sprocket wheel;
Figure 2 illustrates graphically the angular velocity variations; Figure 3 is a schematic representation of the gear and pinion;
Figure 4 is a graphical representation of the angular velocity of a gear of this invention; and Figure 5 displays graphically sprocket torques against time.
Referring to Figures 1 and 2, velocity changes along the line of approach of the track and the vibrations consequent thereon can be ameliorated if the angular velocity, Ws, of the sprocket were to be varied in such a way that, at any angular position, θs, in the cycle of engagement the product r Ws Cos (θs) was constant and equal to the mean approach velocity of the track, Vm. The mean velocity, Vm, is given by:
Vm = 2r • Sin A/2
A/W m rW s in - A/2
I n A / 1 (1) where W is the spprroocckkeett mmeeaann,, oorr eeqquuiivvaalleenntt constant, angular velocity.
Hence thhee ddeessiirreedd aarngular velocity is derived:
= —z\— W sin A 2 s cos θ α m A/2 (2)
The general form of such angular velocity variation is shown in Figure 2 of the drawings. In the practical operation of a real sprocket/ track or sprocket/chain system, dynamic effects and the influence of characteristics of individual system components may be such as to alter the form of the optimum angular velocity variation cycle from that derived above. The optimum form of the variation may be determined empirically or by detailed analysis .
The precise form of the optimum sprocket angular velocity variation so determined is not essential to this invention provided that it conforms to certain constraints , These constraints are limits on the maximum amplitude of the variation of sprocket angular velocity and on the maximum rate of change of sprocket angular velocity relative to its mean.
They exist because of corresponding limits on the maximum amplitude of variation of velocity ratio and the maximum rate of change of velocity ratio in the driving gear linkage. If these latter limits are exceeded gear tooth profiles capable of satisfactory meshing action will not be possible.
The precise values of the limits are dependent on the specific values of a number of gear system para¬ meters and are most unlikely to be exceeded in any track
or chain drive of practical interest.
Combat vehicle sprockets are conventionally directly driven by a simple reduction gear pair in the final drive. It is here that an angular velocity variation in accordance with this invention is introduced.
Using a completely standard input pinion in the final drive it is possible to manufacture an output gear wheel, to drive the sprocket, which will have the desired angular velocity characteristic when the input pinion is driven at constant angular velocity.
The variation in angular velocity of the output gear is produced by continuously varying the pressure angle of this gear around its periphery in accordance with the desired characteristic. . Gear teeth of this kind are known but have never been proposed for use in relation to reducing torque fluct¬ uations in chain drives .
In this regard U.K. Patent 667038 and U.S. Patent 3267763 (Merritt) both disclose the use of varying pressure angles to provide variations in the tangential velocity component to a drive shaft. These proposals have primarily found application in steering mechanisms.
In order to produce a particular gear for a particular vehicle configuration the present invention proposes that the profile of the gear teeth can be deter¬ mined using the following model.
Consider a pinion, P, in mesh with gear G. Assume that over an angular arc of gear rotation, A, which must be such that 2ΪT/A is an integer value, the angular velocity ratio W Q /W of gear and pinion is to be varied through a complete cycle. During this cycle the mean value of W /W must, because of the integer value of 277/A. have the value of -l/R where R is the ratio of tooth numbers N^/Np of gear and pinion.
In order to minimise the need to consider pinion parameters it is useful to introduce the concept of a virtual gear, V, which is a standard gear, co-axial with gear G and also meshing with pinion P.
Hence W = - 1 (3) T R
Assume that the desired angular velocity variation over the arc A is defined by an expression
V W P - f ( V ( 4) where θ is being used to denote angular position.
Hence using 3
G = -R f (Θ Q ) (5)
= -R „ f (θ (6) dt v g
dt = d θ G
-R v f (Θ G )
Integrating from time t = 0 to t = T
where the superscripted symbols refer to values, at the superscripted time.
W is constant and T = θ
or T =
The solution of equation 8 in the general case will yield the relationship between θ and θ p and hence, because of the known linear relationship between θ and θ , the relationship between θ and β . P • P G
The value of the angular velocity ratio for any θp is already known from the function f (θ r ) and it is now possible to define the required tooth profile for the gear,
Referring to the diagram at figure 3, the base circle diameter D of the pinion is known as it is a standard involute gear. The velocity ratio W G / for any Θ G is known and it is equal to the ratio /D G of base circle diameters.
Hence the instantaneous value D p of the gear base circle diameter is known and the common tangent e f can be drawn. Contact between pinion and gear tooth takes place on this line.
•• -3 -
As the position of the pinion θ is known for P all θ p , the point or points C on the pinion tooth or teeth where contact is made with the gear tooth or teeth can be determined. The locus of the point(s) C, relative to the gear, as ,pinion and gear rotate describes the required profile of the gear teeth.
The way in which the basic data describing the gear profile is subsequently manipulated depends on the particular method which is to be used to produce the gear.
As an example of the derivation of equation 8 the relationship given in equation 2 for a sprocket can be used as a basis. This relationship is: w _ I w Sin A/2 s " Cos θ s m A/2
Using the equivalent terminology for the gear driving the sprocket this becomes :
This is now in the form of equation 5 hence f(θ c ) = - 1 sin A/2
R A/2 Cos θ Q
substituting into e quation 8
■■ - 2
defining θ as 0
A/2 sin Θ J
θ- 1 v sin A/2
•A preferred aspect of this invention will now be further described with respect to Figures 4 and 5.
Figure 4 shows the angular velocity of an experimental gear (upper) and its driving pinion (lower) expressed in volts. The high frequency ripple imposed on the required angular velocity characteristic of the gear is the result of machinery inaccuracies and instrumentation characteristics.
Figure 5 is a display of sprocket torques against time measured using an instrumented sprocket drive shaft in a tracked armoured vehicle using a conventional final drive. This tracked vehicle has a 10 tooth sprocket. One revolution of the sprocket corresponds to one cycle of the saw tooth profile which is a plot of sprocket angular position against time over one full turn.
The ten torque pulses which the revised final drive is aimed at reducing are clearly displayed.
Gears according to this invention may be produced from suitable steel alloys using a Fellows type gear shaper. The cutter used will be of the same form as the specific pinion with which the gear of this invention is intended to mesh. The machine itself must be modified
to incorporate into the drive between the workpiece spindle and the cutter spindle, a gear having a velocity ratio characteristic which is the same as or related to that of the gear to be produced.
This "master" gear may be produced by cutting the gear teeth after heat treatment, by using electrical discharge machining on a wire cutting machine.