The Sugar Engineers

Mill Pinions or Crown Wheels

There are normally three mill pinions on a cane sugar mill; one on each of the mill rolls, namely

Top roll
Feed roll
Discharge roll

In mill with and underfeed roll (also known as a the fourth roll) there is often a additional pair of mill pinions. The underfeed roll is driven by a pinion mounted on the non-drive end (also called the pintle end) of the top roll. Whereas the mill crown wheels all have an equal number of teeth the gears driving the underfeed roll are often speed reducing.

the purpose of the mill pinions is to transmit torque from the top roll to the other mill rolls. The feed and discharge rolls normally run at the same rotational speed as the top roll.

Unlike normal gears in a normal gearbox, mill pinions need to able to accomodate a changing centre distance. This is for three reasons

The top roll floats, so the centre distance changes from second to second
The fibre content of the cane changes and to accomodate this, the mill rolls centre distance is checked and adjusted weekly.
The mill rolls wear and to accomodate this wear the centre distcance will change from season to season.

The change in operating centre distance means the pinion tooth profiles need to be sufficiently flexible to accomodate these changes. Contrary to what has been written in Dr Rein's book, Cane Sugar Engineering by Bruce St C Moor it is essential that the tooth profile is involute, no other tooth profile can accomodate a changing centre distance. What is clear though, is that a mill pinion tooth profile can not be in accordance with the two current AGMA tooth profile standards. The AGMA standards are based on fixed centre distances.

AGMA Standard Gears and Mill Pinions
Parameter	AGMA 20° and 25°	Mill Pinion
Tooth Profile	Involute	Involute
Pressure Angle	20° or 25°	16° to 20°
Addendum	1/P	1.1/P
Dedendum	1.25/P	1.5/P to 1.9/P
Fillet Radius	0.3/P	0.6/P to 0.75/P
Circular Tooth Thickness	0.5π/P	0.4π/P to 0.45π/P

While the tooth profile of a mill pinion is an involute curve, this fact is not helpful to th pattern maker in the foundry where the pinion will be cast. The involute curve is approximated by two arcs: one from the base circle to the pitch circle, the second arc extends from the pitch circle to the start of the tip radius of the tooth.