RGB ENGINEERING

INSTALLATION EXAMPLES: UNITED/LOBDELL TWO WHEEL ROLL GRINDER

COMPUTERIZED CROWNER RETROFIT SYSTEM

The photos on the right show the mechanical crowning arrangement on an old Lobdell two wheel roll grinder installed in a paper mill roll shop, as it existed before the retrofit of the RGB Engineering FFG/FFP/SLA computerized crowner system.  A pinion meshing with a rack on the side of the bed drives through a set of change gears to a worm.  The cam, which is really an adjustable eccentric, mounts on the output of the worm gear shaft.  As the carriage traverses along the bed, the cam rotates, the angle of rotation being a function of the change gear ratio.  A horizontal cam follower bar transmits the cam throw to a vertical lever that oscillates a fixed rotation rocker arm.  This rocker arm extends across the carriage, and the lifting action of the eccentric arm operates three pins on each side that engage a slide bar.  These slide bars (one on each side) provide a contact surface for the tilting wheelhead actuator screw.  The actuator screw moves along the top of the slide bar as the sliding base is adjusted in and out.  The wheelhead pivot ratio remains constant due to the design of the slide bar, which lifts uniformly since it is supported by the three pins.  For this machine, the crowning mechanism has been disengaged on the side opposite the cam, which is used only for belt grinding.  In essence, it operates essentially as a one wheel grinder.  A handwheel is attached to the wheelhead actuator screw through a helical gear arrangement.  Although this mechanical crowning device at one time produced reasonably accurate profiles, throughout the years the critical gear meshes and complex mechanism became worn to the point where excessive backlash, stick slip, and erratic operation necessitated that extensive repairs be made.  Rather than spend considerable time and expense to rebuild the existing mechanical system, with associated lengthy downtime and loss of production, the customer elected to purchase the RGB Engineering computerized crowner system.  Reconditioning the original mechanism would have been extremely difficult due to the fact that United/Lobdell is no longer in business, making in-kind parts replacement almost impossible without a complete disassembly of the entire unit to measure and detail the parts. 

The photos below show the same machine after installation of the new computerized crowner retrofit.  It was decided that the best method to achieve the required results with the least amount of cost was to utilize the existing rocker arm portion of the mechanism to actuate the pivoting wheelhead in the conventional manner.  A new Servo Linear Actuator (SLA) was installed as shown, bolted to the same crown drive gear case, with the cam assembly removed.  The SLA consists of a precision preloaded ball screw with a special design nut housing to minimize backlash, driven by a DC servo motor through a harmonic drive reducer.  For this installation, the SLA assembly mounts in a horizontal orientation, so as to apply the force to the rocker arm in the appropriate manner.  A new connecting link was supplied to interface with the standard rocker arm with an improved yoke design, and with a bolted connection to the ball screw housing.  Ringfeder locking elements assure tight fits of the yoke pins.  Since the gravity preload forces eliminate all the backlash in the wheelhead pivot, lifting pins, rocker arm, connecting link, and ball screw, and due to the minimal backlash features of the harmonic drive and timing belt connections, the whole assembly provides a very tight and accurate drive arrangement for the servo system.  The carriage encoder is driven off the same crown rack, which provides an absolute position reference at all times.  There is no need to "home" the control, since this is a true absolute encoder, which remembers its position after a power shutdown.  The LVDT mounts in such a manner, that it measures the tilting motion of the wheelhead in all positions of the sliding base, and provides the servo position feedback signal.  In addition to the standard crowning functions, a precision tilt handwheel is also included, giving the operator the ability to hand feed using the tilt system.  The installation of the complete retrofit system was accomplished in a very short time period, with only about one day of production downtime.