in yet another embodiment the device 10 may be provided with a motorized drive 64 as shown in fig6 the front end 16 of the frame 14 supports a motor 66 the motor 66 includes a drive gear 68 which is in operational engagement with the drive belt 70 the drive belt 70 engages a secondary gear 72 which is attached to front drive wheel 74 the front drive wheel 74 rests on the purlin 6 and upon rotation of drive gear 68 by the motor 66 moves the device 10 along the purlin 6 similarly fig7 shows a motor 66 mounted on the front end 16 of the frame 14 the motor 66 has attached to it drive gear 68 which is in operational engagement with drive belt 70 the drive belt 70 engages opposing drive gears 76 the opposing drive gears 76 are mounted on and operatively connected to opposing drive wheels 82 and 84 the opposing drive wheels 82 and 84 are biased against the sides of top flange 9 of the purlin 6 by tension spring 78 the drive belt 70 is retained in operational engagement with opposing drive gears 76 by idler sprocket 80 drive wheel 84 includes a differential not shown which requires wheel 84 to rotate in a direction opposite that of the associated drive gear 76 the rotation of the drive gear 68 by the motor 66 therefore causes the rotation of opposing drive wheels 82 and 84 and the movement of the device 10 along the purlin 6 other configurations for a motor mounted on the front end 16 of the frame 14 are.