upward position as will be described later herein skid plate 28 is an l shaped member having a horizontal wall 60 and a vertical wall 62 horizontal wall 60 is secured to sidewall 44 of mounting plate 26 by a plurality of bolts 64 bolts 64 are countersunk into the surface of horizontal wall 60 to provide a smooth skid surface for the material being conveyed mounting support arm 30 is also an l shaped member having a vertical wall 66 and a horizontal wall 68 vertical wall 66 mates with and is secured to vertical wall 62 of skid plate 28 by a plurality of bolts 70 mounting support arm 30 thus provides support to skid plate 28 by virtue of horizontal wall 68 bearing against frame 18 to permit u channel 24 and thus wheels 56 to be vertically raised and lowered relative to cavity 46 conveyor 10 is provided with an elongated flexible inflatable hose 72 which is located in cavity 46 between main support frame 22 and u channel 24 hose 72 is positioned on horizontal wall 34 and directly beneath bottom wall 48 thus being confined between walls 34 and 48 hose 72 is connected to a cyclical source of pressurized air not shown such that hose 72 is alternately inflated and deflated when inflated u channel 24 is displaced upwardly thus positioning the upper surfaces of wheels 56 above the level of skid plate 28 in this position the articles or pallets are supported solely by the wheels 56 and move downwardly along conveyor 10 by means of gravity when hose 72 is deflated u channel 24 moves downward under its own weight into the position shown in fig2 in this position the upper surfaces of wheels 56 are positioned below.