the roller bearings suitable seal rings 60 may also be utilized to retain lubricant in the bearings and to exclude dirt therefrom in order to secure the arbor 36 to the mounting plate 30 a plurality of fasteners or threaded bolts 62 are utilized which extend through the elongated slots 34 and are engaged in a corresponding plurality of threaded bores 64 in the arbor these fasteners are oriented in a direction parallel with the arbor 39 s axis 38 and with the heads thereof spanning the opposite sides of the slots as best shown in fig2 so that as they are screw threadably tightened the end mounting surface 37 of the arbor is clampingly engaged against the mounting surface 32 of the mounting plate pursuant to the present invention an adjustable side thrust force reaction device 66 advantageously provides a reaction to resist the forces which are directed laterally outwardly or to the left when viewing fig1 from the inner upright 16 and through the roller 44 and the arbor 36 such a device includes an abutment bar or pull in block 68 which is secured to the mounting plate 30 in rearwardly projecting relation therefrom the bar has a bore 70 defined therethrough which opens outwardly on a longitudinally oriented vertical face 72 and on an inner reaction face 74 parallel thereto a fastener.