Constant Velocity Assured by
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Bendix Tracta Universal Joint
A Simple Mechanism for Coupling Shafts Having a Wide Angularity, in which all Mathematical Rules are Observed and Large Bearing Surfaces Afforded
rURRENT tendencies in design lead, in certain instances,
the use of comparatively short transmission shafts and considerable variation in the angle through which power must be transmitted at the ends of those shafts. Examples are independent rear suspension and front-wheel drive, in which the steering motion is an obvious complication for the transmission designer.
These facts, amongst others, lend special interest to the Tract:. joint which is manufactured, under licence, in England by Bendix, Ltd., Tyseley, Birmingham. This device is mechanically simple; it has large plain surfaces to carry its loads, and it does ensure that each of the two shafts it couples shall rotate at the same velocity as the other.
It is appropriate to recall that the ordinary Hooke's joint causes a spee4 variation between driving and driven shafts of nearly 30 per cent, if the shafts be at an angle of 30 degrees, and this variation occurs four times per revolution. Little imagination is needed to picture the heavy loads that can be caused by the resulting ac:celerations if the shafts he rotating at considerable speed.
It is stated that three conditions are necessary to ensure constant velocity between two shafts coupled at an angle. The shaft axes must always meet at a given point. The two sections of the coupling must be symmetrical about the plane which bisects the angle formed by the two shafts. The halves of the coupling must be perpendicular to the plane which contains the axes of the two shafts.
The end of each shaft in a Tracta joint is forked. Perhaps, a clearer mental picture is given by saying that a flat ring is formed with a diameter in line with the shaft axis and that rather less than half the ring is cut away. Connecting these forks are two substantial members.. of peculiar shape. Each has a groove into which one shaft fork fits, and each member is thus free to turn 'about the axis of the ring from -which this fork is formed.
One of these members has a somewhat similar but concave groove in its opposite surface and in a plane at right-angles to the plane of the first groove. Fitting into this groove or recess there is a corresponding projection on the other member. Consequently, one member can slide on the other within the plane of that groove and projection, but they Must turn together if one of them moves in such a direction as to rotate that plane. .
To complete the Tracta joint there is a housing which may take several forms. Probably, the most common application in road-vehicle design would be a pair of shafts of variable angle and with the housing rotating with these shafts. In that case, the housing consists of two principal members, one attached to each shaft. One has a spherical exterior and the other a spherical interior fitting nicely into the former. The centre of the sphere, of course, coincides with the point of intersection of the shaft axes.
In a somewhat similar arrangement, the housings do not rotate but carry bearings in which the shafts revolve. Other applications are to shafts operating at a fixed angle to one another and it is interesting to note that the Tracta joint will also function in the manner of an Oldham coupling, so that it can be used to connect two shafts which are parallel but not in line.
.Apart from the transmission of power in numerous engineering fields, the Tracta joint is also finding application in various control mechanisms where angular movement of a wheel or lever must be reproduced accurately at _the other end. An informative and well-illustrated booklet dealing very fully with this joint can be obtained from Bendix, Ltd.