arm 26 more specifically the third joint assembly 38 includes a shaft 68 having a lower shank portion 68a which is fittingly retained inside the housing 42 and an upper portion 68b which has a cross section similar to that of the small diameter portion 48b of the shaft 48 a friction cone 70 is keyed onto the portion 48b and has a diameter which tapers inwardly from the bottom to the top thereof as viewed in the drawing another friction cone 72 is keyed onto the portion 48b and has a diameter which tapers inwardly from the top to the bottom thereof as viewed in the drawing a body 74 is securely retained in the arm 26 by screws 76 and has a bore formed with tapers conjugate to those of the cones 70 and 72 with the cones 70 and 72 fitting into the lower and upper tapers of the bore of the body 74 respectively the upper end of the portion 68b of the shaft 58 is threaded a nut 78 is screwed onto the upper end of the portion 68b thereby urging the cone 72 downwardly through an intervening washer 79 and urging the cone 70 upwardly into the tapered bore of the body 74 a wave spring 80 is provided between the upper end of the shank 68a and the lower end of the cone 70 the cones 70 and 72 are thereby pressed into the body 74 with the mating tapered surfaces thereof being frictionally pressed together the body 74 rotates integrally with the arm 26 whereas the cones 70 and 72 rotate integrally with the shaft 68 the frictional resistance between the mating tapered surfaces of the cones 70 and 72 and body 74 provides a selected value of resisting torque.