the end 26 u2032 of drive screw 26 includes a threaded aperture 28 for receiving a fastening screw 29 thus end 26 u2032 of drive screw 26 is rotatably received in an aperture 37 of gear box housing 36 and is rotatably and vertically held within aperture 37 by fastening screw 29 and washer 31 fig3 gear box 36 conventionally includes an internal drive gear not shown which engages drive gear 42 fixedly mounted to end 26 u2032 of drive screw 26 for rotating the drive screw 26 in response to actuation of motor 34 in opposite directions for extending and retracting the inner leg 14 with respect to the outer leg 16 the motor mounting plate 38 includes in a preferred embodiment shown three equidistant downwardly extending pins 50 52 and 54 which are axially aligned and spaced from the screw jack end 26 and extend downwardly as seen in fig1 and 3 and are received by three equally spaced elastomeric members such as grommets 60 62 and 64 mounted in apertures 70 72 and 74 formed in surface 23 of base 20 as best seen in fig2 pins 60 62 and 64 are preferably tapered at an angle of up to about 10 u00b0 as seen in fig3 to readily fit in the apertures of polymeric grommets 60.