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The Vernisse Coupling for Tandem Engines A Brief Note on the Ingenious Constant-Torque Joint Developed by the Arsenal de l'Aeronautique N the articles on the VB-10 tandem-engined The coupling (FIGS. 1 and 2) consists essentially single-seater and the Arsenal de l'Aéronautique of a driving plate 1 and a driven plate 2. On the I driving plate there is a series of spigots, 3 with the Vernisse coupling has been mentioned*. spherical driving bosses. These bosses rest in self- The desirability of a constant-torque coupling was realized as long ago as 1937 and work was aligning bearings 4 carried in the sleeves 6, which begun on the design in 1939. The idea was to in turn rest in the self-aligning bearings 5 mounted develop a coupling primarily to enable engines to on the driven plate. The result of this is that each be mounted in tandem so that they would drive spigot is a floating assembly free to take up any co-axial airscrews, but also to provide a shaft relative position within the value limited by the joint that would take up structural deflexions for chosen dimensions of the parts. Since the axes of the two bearings (4 and 5) are held parallel by extension drives of single engines. simultaneously to the masses represented by the the sleeve 6 and perpendicular to the plane The design of a transmission was governed by same numbers as their moments of inertia I and bisecting the angle between the driven and driving two principal considerations; the effect of I . shafts the condition of constant velocity is satis deformation of the structure, and the bending The calculation is simplified by reducing the fied and the driving torque is equal to the torque and twisting stresses in the shafts. whole system to an equivalent system of constant transmitted. polar moment of inertia over the whole length of These considerations led to the design of two the shaft. Deformation of the Structure types of constant velocity coupling: one with A first test of the behaviour of the constant off-set, or oblique, driving spigots and the other To take this deformability into account, the velocity couplings was undertaken with a with concentric spherical joints. The first proved transmission was provided in the form of several Hispano-Suiza 12 Y engine, driving an airscrew under test that the angularity of the planes of the sections, normally aligned, and connected by at a distance of 6 metres by means of four sections driving and driven members on the central means of deformable couplings in such a way as of shaft with a deflexion of 1 deg. 30 min. at each member of the coupling produced a relatively to allow some latitude in alignment, for the fol coupling.† This was followed by coupling two pronounced rate of wear. On the other hand, lowing reasons: consider a shaft in several sec Hispano-Suiza 12 Z engines, giving 2,700 h.p. as couplings with concentric sleeves avoided this tions driving an airscrew, each section being able used in the VB10, and also two Jumo 213 A disadvantage. These consist of sleeves linked to make a small angle with the preceding section. engines, giving 4,000 h.p.† and finally with two to the driving plate by spherical bearings mounted If the transmission consists of a classical candan- Arsenal 24H engines, in line, giving 8,000 h.p. in spherical housings in this plate and sliding over joint system, it is known that the ratio of the The possibilities of this coupling for large air the sleeve and to the driven plate by spherical speeds ω/ω varies between cos α' and 1 /cos α'. 0 craft with the engines buried in the wings and jointed pins mounted in this plate and sliding For example, assuming that the angle between completely accessible in flight are obvious and inside the sleeve—in other words a reversal of the two adjacent sections is as much as 3 deg., we projects based on this principle are already under the original coupling which was shown in FIG. 1. have development in France. The disks supporting the sleeves are formed by cos α' = 0·9986 and l/cos α' = 1·0014 The weight per horsepower of these first tandem arms, rigid or articulated, according to the angle from which arrangements has been found to be: between the sections of shaft to be coupled. ωmax = 1·0014ω ωmin = 0·9986ω . 640 g/h.p. for two Hispano-Suiza 12 Z engines of 0 0 Since the shafts are hollow tubes they transmit The inertia torque concerned is 2,700 h.p. the driving torque by shear in torsion. I = Iairscrew dω/dt. 525 g/h.p. for two Jumo 213 engines of 4,000 h.p. Moreover, each section has its own bending Further, the instantaneous speed of transmission 510 g/h.p. for two Arsenal 24 H engines . oscillation frequency as a function of its inertia of such a coupling is given by the equation The weight of the complete transmission repre and the distance between its supports. The sents not more than 14 per cent of the total power critical rotational speed at which the natural unit weight (without airscrews) or vibration frequency of the shaft synchronizes 340 kg. for the 12 Z tandem giving 2,700 h.p. with the frequency of the rotational impulse is β being the angle of rotation. Weight of engines alone 1,390 kg. given by the equation, The variation of this speed is obtained by Total weight of tandem 1,730 kg. differentiation: †[See AIRCRAFT ENGINEERING, June, 1947, p. 182, Fig. 14. Where N = r.p.m.; g = 9·81 ; and feq = the equiva If the following values are substituted: lent deflexion given by the formula n = 1300 r.p.m. or 136 radians/sec. α' = 3°, sin α' = 0·0523 sin β = 0·5 where f is the deflexion perpendicular to the an d Iairscrew = 4· 5 point of application of the load ρ. we obtain The inertia and the distance between the sup ports of the shafts are determined in such a way that N is either very small or very large with respect to the normal engine speed, as, if the = 228 kgm. oscillations synchronize near the usual speed of For a 12-cylinder engine, of which the mean rotation, the amplification of the vibrations might torque is about 716 kgm. and which may vary by lead to failure by resonance. ±236 kgm., the sinusoidal torque due to the The critical torsional velocity, which would cardan joint and the airscrew inertia being cause failure in shear at resonance, is given by approximately ±228 kgm., would give rise to a the expression periodic torque of which the extreme variations might amount to ±470 kgm. Since this type of joint was not acceptable the coupling of the two engines was effected by constant velocity coup lings which eliminate the amplification of the where θ is the angle of equivalent torsion, of eq torque by angular displacement. which the value is * See 'The Arsenal de l'Aeronautique VB-10C1', AIRCRAFT ENGINEERING, February, 1947, pp. 43-45 and 'The Arsenal de T and T being the turning moments applied l'Aeronautique, Chatillon-sous-Bagneux', June, 1947, pp. 178-183. i1 i2 July 1947 217
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1947
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