Rotary drives

A rotary drive mechanism has an input shaft with a first crank, mounted parallel to an output shaft, the input shaft being offset from the output shaft by a distance equal to the length of the first crank, the output shaft having a second crank of greater length than the first crank, the first and second cranks are pivotally connected to opposite ends of a link, interlocking formations are provided to prevent rotation of the second crank and link independently of the input shaft when the second crank and link are parallel to one another.

BACKGROUND TO THE INVENTION 
The present invention relates to a rotary drive and in particular to a 
rotary drive suitable for use with rotary valve mechanisms in which a 
intermittent drive is desirable. 
It has been proposed hitherto to use rotary valves for internal combustion 
engines, but in order to seal the rotating valve member on the high 
pressure (i.e. compression and combustion) strokes of the engine, complex 
gas seals have been required. Furthermore, such systems result in high 
frictional loads with consequent reduction in efficiency and high wear 
rates. In order to reduce the problems of high frictional loads, it has 
been proposed to reduce the speed of rotation of the valves on the high 
pressure strokes of the engine, using a differential drive gear 
arrangement. Such systems are however relatively complex and the space 
required for the differential drive gear arrangement will substantially 
increase the overall height of the engine. 
The present invention provides a rotary drive of relatively simple 
construction, which will significantly reduce the speed of rotation of a 
rotary valve during high pressure strokes of the engine. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention, a rotary drive mechanism 
comprises an input shaft having a first crank, an output shaft mounted 
parallel to the input shaft but offset radially therefrom by distance 
equal to the length of the first crank, said output shaft having a second 
crank the length of which is greater than the length of said first crank, 
the first and second cranks being pivotally connected to opposite ends of 
a link, the link being the same length as the second crank, and interlock 
means being provided between the first crank and at least one of the 
second crank and link, to prevent rotation of the second crank and link 
independently of the input shaft, when the second crank and link are 
parallel to one another. 
With the rotary drive disclosed above, two revolutions of the input shaft 
are required to rotate the output shaft by one revolution. Furthermore, a 
variable speed drive is provided. 
The actual variation in rate at which the input shaft drives the output 
shaft depends on the difference in lengths of the first and second cranks. 
The smaller the difference in length, the greater the variation in speed 
of the output shaft. There is, however, a physical limitation to the speed 
differential that can be achieved as the system has to be able to provide 
the appropriate acceleration to the output shaft and components associated 
with the output shaft. With small differences in length between the first 
and second cranks, the acceleration required will be extremely high and 
for most applications will be impractical. Nevertheless, with the device 
disclosed above typically up to 95% of rotation of the output shaft 
corresponds to one rotation of the input shaft, while for 180.degree. 
revolution of the input shaft, there will be very little movement of the 
output shaft. 
This drive system is particularly suitable for rotary valve mechanisms, the 
drive being arranged so that the rotary valve is only driven slowly during 
the high pressure strokes of, for example, an internal combustion engine. 
This significantly reduces wear on the valve rotor and port seals. 
Furthermore, the two to one drive ratio is ideal for four stroke engines 
where the valves are opened once for every two cycles of the engine. 
Furthermore by rotating the valve rotor slowly while the ports are closed, 
the ports may be made larger and thus more efficient.

DESCRIPTION OF A PREFERRED EMBODIMENT 
As shown in FIG. 1 a rotary drive comprises an input shaft 10 having a 
crank 11 mounted thereon for rotation therewith. An output shaft 13 is 
parallel to the input shaft 10 but offset radially therefrom by a distance 
equal to the length L.sub.1 of the crank 11. A second crank 14 is mounted 
on output shaft 13 for rotation therewith. The two cranks 11 and 14 are 
pivotively connected to opposite ends of a link 16 by means of crank pins 
17 and 18 respectively. 
A formation 20 is non-rotatably mounted on the end of crank pin 17, so that 
it is located between the link 16 and crank 14. Said formation 20 has a 
flat face 21 which is transverse to the crank 11 and offset from the axis 
of the crank pin 17 towards the input shaft 10. 
A first formation 22 is provided on the second crank 14 offset from the 
axis of the output shaft 13 towards crank pin 18. As better illustrated in 
FIG. 2, face 23 of the formation 22 is contoured to conform with the path 
of the face 21 of formation 20, as the crank 11, link 16 and crank 14 move 
together to the position illustrated in FIG. 1, so that in that position 
the contoured face 23 will abut the flat face 21 which will prevent crank 
14 and link 16 pivotting about the aligned axes of the output shaft 13 and 
the crank pin 17. 
A second formation 25 is provided on crank 14 on the opposite side of the 
axis of the output shaft 13 to formation 22. Formation 25 has a flat face 
26 which is offset from the axis of the output shaft 13 by the same amount 
as the face 21 of formation 20 is offset from the axis of crank pin 17. As 
the input shaft 10 completes one revolution and link 16 and crank 14 move 
towards the position illustrated in FIG. 3, he face 26 will move into 
engagement with face 21 so that in the position illustrated in FIG. 3, the 
faces 21 and 26 will abut, again preventing rotation of the crank 14 and 
link 16 about the aligned axes of output shaft 13 and crank pin 17. 
Alternatively, the interlock means may be provided between the crank 11 
and link 16 or the interlock for one position may be provided between 
crank 11 and link 16 while the interlock for the other position is between 
the crank 11 and the crank 13. 
The drive mechanism illustrated in FIGS. 1 to 3 is suitable for use in 
driving the rotary valve of an internal combustion engine. As illustrated 
in FIG. 4, a valve disc 30 is mounted on the output shaft 13 which passes 
through the cylinder head 31. The disc 30 overlies an exhaust port 32 and 
an inlet port 33 in the cylinder head 31, a segment 34 of the disc 30 
being removed so that as the disc 30 rotates it will open and close ports 
32 and 33 in turn. 
The exhaust and inlet ports 32 and 33 are positioned so that there is a 
space 35 therebetween which is greater than the segment 34 removed from 
disc 30. An ignition device 36 is located through the cylinder head in the 
portion thereof defined by space 36. 
FIGS. 5A to 5H illustrate the movement of the valve disc 30 and the opening 
and closing of exhaust and inlet ports 32 and 33 as the input shaft 10 
rotates through 90.degree. steps. 
As illustrated in FIGS. 5A to 5H, as the input shaft 10 moves through 
90.degree. from the position illustrated in FIG. 5A, there is very little 
rotation of the output shaft 13 and disc 30 and the exhaust and inlet 
ports 32 and 33 remain closed as illustrated in FIG. 5B. After a further 
90.degree. rotation of shaft 10, the output shaft 13 is beginning to speed 
up and the exhaust port 32 is just beginning to open as shown in FIG. 5C. 
During the next full revolution of shaft 10 from the position illustrated 
in FIG. 5C to that illustrated in FIG. 5G, the output shaft 13 and disc 30 
are driven relatively rapidly to open exhaust port 32 and inlet port 33 in 
turn. From the position illustrated in FIG. 5G, the valve rotor 30 begins 
to slow down and the inlet port 33 is closed by the time the input shaft 
10 reaches the position illustrated in FIG. 5H. The shaft 13 and valve 
disc 30 are only rotated by a small amount as the input shaft rotates by 
90.degree. from the position illustrated in FIG. 5H back to the position 
illustrated in FIG. 5A. 
As shown in FIGS. 5A to 5H, approximately 75% of rotation of the valve disc 
30 occurs while the input shaft 10 is rotating through one revolution, 
from the position illustrated in FIG. 5C to that illustrated in FIG. 5G. 
The exhaust and inlet ports 32 and 33 remain closed for over 180.degree. 
of a revolution of the input shaft 10, from the position illustrated in 
FIG. 5H to that illustrated in FIG. 5B. 
During the period for which the exhaust port 32 and inlet port 33 are 
closed, even though the input shaft 10 is driven at a constant speed, the 
valve rotor 30 will only be rotating slowly. This portion of the cycle, is 
timed to occur during the high pressure strokes, that is the compression 
and combustion strokes of the engine. During this period, the disc 30 is 
allowed to engage a seating area on the cylinder head 31 to provide a 
tight seal. Also in this portion of the cycle, the segment 33 exposes the 
ignition device 36 to the fuel/air mixture in the combustion chamber, so 
that ignition can take place. 
Means may be included in the drive mechanism, for example a helical drive 
element, which will lift the valve disc 30 away from the seating area of 
cylinder head 31 as the valve disc 30 is accelerated by the drive 
mechanism, and allows the valve disc 30 to move down into engagement with 
the seating area, as the valve disc 30 is decelerated by the drive 
mechanism. This helical drive element could be a multi-start thread 
between the output shaft 13 and disc 30. 
While the above embodiment provides a dwell period in which the drive to 
the valve rotor 30 is substantially reduced and the exhaust and inlet 
ports 32 and 33 remain shut, of approximately 180.degree., this dwell 
period may be adjusted by adjusting the relative lengths of the crank 11 
and the crank 13 and link 16, to suit the requirements of the engine. 
In multi-cylinder engines, each cylinder will have a rotary valve with a 
rotary drive of the form disclosed above. The input shafts 10 of the 
rotary drives will all be parallel to one another. These input shafts 10 
may be connected to a common drive from the crank shaft of the engine, 
each drive being connected in appropriate phased relationship with the 
crank shaft. 
According to one embodiment, a common input shaft driven by suitable means 
from the crank shaft may be arranged transverse to the input shafts 10, in 
similar manner to the overhead cam shaft of a conventional engine with 
poppet-type valves. Drive may then be transmitted from the common shaft by 
suitable means, for example bevel gears, which will transmit the drive 
through 90.degree.. 
In an alternative embodiment, as illustrated in FIG. 6, the cranks 11 of 
all the rotary drives for rotary valves 30 serving cylinders 40a to 40d 
are defined by a common plate 41. 
The plate 41 is drivingly connected to the engine crank shaft by means of a 
input shaft 42 which is parallel to the axis of rotation of the rotary 
valves 30. An eccentric crank 43 on the input shaft 42 engages the plate 
41 to drive it in orbital manner. A pair of idler shafts 44 with cranks 45 
of the same eccentricity as the input shaft crank 43, are provided at 
opposite corners of the plate 41 to stabilize the orbital motion thereof. 
The eccentricity of the cranks 43 and 45 is equal to the length L1 of the 
cranks 11, so that during orbital motion of the plate 41, any point 
thereon will describe a circle of diameter L1 equal to the circular path 
described by the crank pivot 17 as the rotary drive described above 
rotates the rotary valve 30. 
A series of drive holes are provided in the plate 41, in which the crank 
pins 17 of the rotary drives associated with the different cylinders 40a 
to 40d, may be engaged. Alternatively, the crank pins 17 may be provided 
on the plate 41. The crank pins 17 pivotively connect links 16 to the 
plate 41, the links 16 being pivotively connected to cranks 14 associated 
with output shafts 13, in the manner described above. 
As the plate 41 is driven to prescribe an orbital motion, the crank pins 17 
will be driven about circular paths (shown in broken line in FIG. 6) of 
diameter L1 with a centre 10' which is equivalent to he axis of rotation 
of input shaft 10 of the rotary drive described with reference to FIGS. 1 
to 5. The crank pins 17 will thus drive link 16, crank 14 and the output 
shaft 13 in the manner described above. 
With the engine configuration described above, the firing order would 
typically be 40a; 40c; 40d; 40b and the phased relationship between the 
cylinders 40a to 40d would be 0.degree.; 90.degree.; 270.degree.; 
180.degree.. In order to achieve appropriate phase relationship between 
the rotary valves 30, the valve ports 32 and 33, rotary valves 30 and 
rotary drives for cylinders 40b and 40c are disposed at 180.degree. 
relative to those of cylinders 40a and 40d; the position of the drive 
holes 47 on plate 41 being altered correspondingly, so that while the 
circular paths of each drive hole passes through the axis of the 
associated input shaft 13, the circular path described by those drive 
holes associated with cylinders 40a and 40d on the opposite side of said 
axes from the circular paths described by the drive holes are associated 
with cylinders 40b and 40c. The rotary drives associated with the 
cylinders 40a and 40b are then connected to the plate 41 by engagement of 
the crank pin 17 in drive hole, so that they are one rotation 
(360.degree.) out of phase with the rotary drives associated with 
cylinders 40d and 40c respectively. 
In this manner, as illustrated in FIG. 6, when the cylinder 40a is just 
about to fire, the rotary drive will be in the position illustrated in 
FIG. 5A, both ports 32 and 33 being closed by rotary valve 30; cylinder 
40b will be beginning its exhaust stroke, its rotary drive being in the 
position illustrated in FIG. 5C with exhaust port 32 just opening; 
cylinder 40c will be beginning its compression stroke, its rotary drive 
being in the position illustrated in FIG. 5G with inlet port 33 about to 
close; and cylinder 40d will be beginning its induction stroke, its rotary 
drive being in the position illustrated in FIG. 5E, the exhaust port 32 
just closing and inlet port 33 just opening. 
FIGS. 7 to 9 illustrate an alternative interlock arrangement, which may be 
used in the embodiment illustrated in FIG. 6, to prevent crank 14 and link 
16 pivotting about the axes of output shaft 13 and crank pin 17, when 
these are aligned as in the positions illustrated in FIG. 6 for cylinders 
40a and 40d. 
With this interlock arrangement, for each rotary drive, a first formation 
50 is provided on the side of plate 41 adjacent the rotary drive, this 
formation 50 defining a flat face 51 which will engage the flat face 52 of 
a flange formation 53 on the end of crank 14 adjacent output shaft 13, 
when the rotary drive is in the position illustrated for cylinder 40a. 
A pin formation 55 is also provided on the side of plate 41 adjacent the 
rotary drive. A cup formation 56 is provided at the end of crank 14 
adjacent crank pin 18, the pin formation 55 being positioned on the plate 
41 and the cup formation 56 being contoured such that as the crank 14 
moves relative to the plate 41, the pin formation will move into and out 
of engagement with the cup formation 56, so that when the rotary drive is 
in the position illustrated for cylinder 4d, the pin formation 55 will be 
engaged within the cup formation 56. 
Various modifications may be made without departing from the invention. For 
example, while the drive mechanism is particularly suitable for driving 
rotary valves, it may be used to drive other elements where a variation in 
the speed throughout the rotation of the element is required. Although the 
valve rotor described above is in the form of a disc with a single 
aperture, discs, cones or cylinders with one or more apertures, or rotors 
having vanes which extend radially to sweep one or more ports may be used. 
Rotary valves of the type disclosed may also be used to control flow to 
any number of ports. 
While in the embodiment illustrated in FIG. 6 the orbiting plate 41 is 
stabilised by means of idler shafts 44, other means may be used for this 
purpose, for example the plate may be linked to a support member by means 
of an Oldham coupling. When used with valve gear, the rotary drive 
mechanism will give the required 2:1 drive ratio, the drive connection 
from, for example the crank shaft, should consequently be direct. However 
if other drive ratios are required this may be achieved by varying the 
ratio of drive from the crank. For example, in the embodiment illustrated 
in FIG. 6, the shaft 42 may drive plate 41 in orbital manner via an 
eccentric gear arrangement which will provide the necessary variation in 
drive ratio.