Throwing a different pitch at linear drive complexity and downtime
By Mark Wilson
In industrial processes, dedicated reciprocating systems typically operate using the motion of a threaded drive nut on a ball screw. The reversal of linear travel direction is accomplished through the use of switches or sensors as well as a motor that reverses the screw's rotational direction. However, this approach requires a fair bit of complexity and introduces significant downtime for maintenance.
Moreover, these components are manufactured--that is, the threads are machined-- to a particular pitch size, and are therefore fixed-pitch components. Theoretically, one could adjust the motor speed so that the drive nut travels more or less quickly over the same fixed-pitch distance of the ball screw or threaded shaft. In fact, the only way to vary the linear output speed in such systems is to control the motor speed (a potentially complex and expensive solution), since the pitch is pre-set.
Speed and pitch aren't the same
The terms "speed" and "pitch" are sometimes mixed-up in discussions of linear motion. It is helpful to differentiate the terms speed and pitch, by using the formula:
Linear output speed=pitchXshaft speed (motor r/min.)
In linear motion circles, the term "pitch" generally refers to the linear output produced by one revolution of the shaft. Pitch defines how far the drive nut travels on one revolution of the shaft, whereas shaft speed defines how fast the motor shaft is turning. For example, a shaft with a quarter-inch pitch turning at 60 r/min. will generate a linear output speed of 15 in./min.; while a shaft with a half-inch pitch turning at the same 60 r/min. will generate a linear output speed of 30 in./min.
Conventional thinking has it that pitch is a fixed factor, but what if that weren't the case? What if you could control and adjust the pitch--that is, vary the amount of distance the drive nut travels for every revolution of the shaft, without changing the shaft speed (motor r/min.)? If that was possible, adjustable speed motors and potentially complex controls would become unnecessary.
In fact, it is possible to control linear output speed without changing motor speed--by simply adjusting the pitch setting on the traverse.
An unthreaded drive shaft passes through the center of a series of three or four ring bearings, each with a specially contoured inner surface. The rings are situated in a cube-shaped housing.
A single speed, unidirectional motor drives the system. Users can select the direction of rotation. When the shaft rotates, the contoured inner surface of the rings bears against the smooth shaft surface, causing linear movement of the housing and attached load. In many cases, pitch is adjustable "on the fly" simply by changing the angle of the rings while the drive is moving.
Absence of the threads eliminates clogging. Drive will slip, not jam, thereby preventing damage to costly production equipment. No shaft bellows required.
Automatic reversal trigger is mechanically "flipped" to change the angle of the rolling ring bearings to their exact opposite hand. This enables backlash-free reversal of traversing direction.
Free movement lever permits manual positing of the nut without turning the system on.
How is it possible to control linear output speed by adjusting pitch, without affecting the drive shaft rotation speed? Adjusting the pitch of a machined ball screw and threaded nut, of course, is impossible. But what if you were to replace the threaded nut and ball screw with a smooth shaft, then find a means to change the distance travelled by the drive nut per revolution of the shaft while the motor turns at a constant speed? It would be possible to variably control the linear output speed of your load, without changing drive shaft speed.
By applying its patented Rolling Ring approach to reciprocating linear actuator technology, Amacoil Inc., has done exactly that. Rolling Rings are designed to address a smooth shaft at an offset angle, so that they operate like a ball screw and a threaded nut. In effect, they create variable "invisible threads" on an unthreaded shaft.
The Rolling Ring principle is demonstrated by Amacoil's RG linear actuators, which are designed for reciprocating applications such as spooling, slitting, or spraying. In RG operation, an unthreaded drive shaft passes through the center of a series of three or four specially contoured (rolling ring) bearings. These are situated in a load-bearing housing. As the motor-driven shaft rotates, the contoured surface of the rings bear against the smooth surface of the shaft, causing the rings and housing to move in a linear direction. Payloads mounted on the housing move, naturally, with the assembly.
By adjusting the angle at which those rings address the shaft, the pitch and linear output speed of the payload housing can be variably controlled without affecting the motor speed. You can have the motor turn at a constant speed and still change the drive nut's linear output per revolution of the shaft (pitch), because the pitch is a function of the rings' contact angle, rather than being fixed by a machined thread pitch.
With the Rolling Ring method, pitch adjustments can be made using a simple pitch indicator dial on the linear actuator. The RG linear actuators are designed with either 50 or 100 discrete points on the dial, from minimum to maximum. Each increment denotes either one fiftieth or one one-hundredth of the total available range between the minimum and maximum available pitch.
The mechanical simplicity of reciprocating motion using this technology extends to the reversal of travel direction, as well. Instead of sensors, simple mechanical reversal stops "flip" the rings' orientation angle, changing the traverse direction instantaneously without backlash. Because of their ability to reverse direction without backlash, the Rolling Rings permit smooth alternating left-right motion, or traversing. The stroke distance from reversal point to reversal point can also be changed, without changing any other parameters, by moving the location of the reversal stops by hand. By design, the added expense of pre-loaded nuts is not required.
Synchronize and simplify plant processes
Standard RG drives permit the incremental adjustment of pitch using the pitch-control dial. It is also possible, using drives fitted with optional infinitely variable set screws, to precisely match traverse speed to the speed of already existing processes, such as the cutting of continuously fed materials by travelling cutting devices. By using a common single motor drive for both the Rolling Ring actuator and the material feed, synchronization will be maintained even if the speed of material feed varies.
When using a ball screw and a threaded nut to generate linear motion, it is usually necessary to cover them with a bellows assembly as protection against dirt and debris. But the smooth RG shaft accumulates little dirt or debris and therefore there is usually no need for such costly and cumbersome protection. This can also help reduce maintenance and changeover downtime, by eliminating the need to remove and reinstall the bellows assembly.
In the event of an overload, the Rolling Ring drive nut "slips" on the drive shaft, instead of jamming. Threaded shaft/nut combinations tend to "churn" when overloaded, often damaging themselves as well as associated system components.
By putting a new twist on an old pitch, Rolling Ring systems permit linear output speed to be varied without the use of expensive variable speed motors or elaborate gearing.