Roto-Lok® Technology

Downloadable Files Roto-Lok® Technology

The Roto-Lok® Rotary Drive is an elegant, simple, technology that utilizes the averaging effect of many cables wrapped around a drive capstan and an output shaft. It was originally invented as an inexpensive way to rotate large observatory telescopes accurately and smoothly.

The driven capstan and output shaft (drum) are connected by multiple precision cables wound in a figure eight pattern around the shaft and capstan. The cables share the load, each with a tension spring at one end and a fixed mounting point at the other. The drive carries torque through the friction of the cables on the drum and capstan, and also through cable tension. The springs accommodate wear and keep the proper design tension throughout the length of the cable. Because the Roto-Lok® drive is constantly preloaded in both directions, there is absolutely no backlash. Unlike gears that wear and degrade performance, the springs keep the proper tension indefinitely; resulting in "like new" performance throughout many millions of cycles. The Roto-Lok® drive requires no lubrication, and is virtually maintenance free.

Roto-Lok® Rotary Drive capstans and output shafts typically have smooth cylindrical shapes that are relatively easy to produce. Either may be grooved to improve cable tracking, and can include mounting, weight-reduction, and other necessary features. The arrangement of the Roto-Lok® drive places the capstan and output drum close together, minimizing the unsupported free span of cable, and maximizing the drive’s coupling stiffness. The many cables serve to average the rotation rate so that imperfections, dirt or other slight irregularities on a single cable or drum do not have a significant effect. This results in superb drive smoothness with no cogging or drive rate irregularity.

Roto-Lok® drives provide useable ratios from 1:1 to 20:1 or higher. Because of their extremely high efficiency (in excess of 99%), they can also be back driven to provide a speed increase rather than speed reduction.

Among the many advantages and benefits of the Roto-Lok® drive:

• Backlash free & high stiffness. The load bearing elements (cables) are statically tensioned to increase the stiffness of the drive. In a gear drive, such tensioning would create friction and shorten the useful zero-backlash life of the gears.
• Multiple spring-loaded cables carry the load and provide superb smoothness by averaging the work done by each individual cable. This virtually eliminates "cogging" found in traditional gear drives.
• Cable wear from sliding or stretching is minimal. The cable simply picks up or lays down on the capstan and drum.
• High efficiency that is comparable to the loss created by the use of a high quality bearing. Because the drive is stiff and efficient, smaller motors, wiring and power supplies can be used.

Application of the Roto-Lok® rotary drive typically provides the following benefits:

• Reduces motor power proportional to drive ratio.
• Lubrication required only in bearings.
• The drum can be designed with a large through-hole to accommodate optical beams or cables.
• Either the drum or the capstan can be fixed. Fixing the drum often results in no relative motion between the motor, capstan, and payload. This improves the packaging of some systems.
• Conformable form factor. The Roto-Lok® Drum can be integral to the mechanical structure, and can be shaped as an arc segment, or a complete circle.
• Conformable shapes often reduce weight when compared to other types of drives.
• The drive works equally well underwater, or in dusty or oily environments.
• It can be used in clean rooms, food or pharmaceutical processing or medical environments.
• Shock and vibration resistant.

What Can’t the Roto-Lok® Technology Do?

As with all technologies, the Roto-Lok® rotary drive is not perfect. It has its own unique set of limitations, including the following:

• Continuous rotation is not possible. Both ends of the cable must be fixed to the drum.
• Ranges of "n" times 360 degrees are possible, however, at these ranges drive ratios are limited by the cable layout on the drum.
• For the best positional repeatability over time, the position instrument must be on the output shaft. At a micro-level, the cables can "creep" around the capstan. In many systems, this condition is reduced or eliminated by increasing the number of cable wraps around the capstan.
• Similar to "friction" drives, the drive ratio is not absolute. Small variations in the output shaft, capstan, and cable diameters as well as variations caused by thermal expansion/contraction can affect the drive ratio.
• For a given drum diameter, the required drum height will increase as the range of motion increases.
• Torsional stiffness and its effect on drive performance is a function of many factors. The stiffness is reduced as a function of increased load torque. A drive with acceptable stiffness with one payload may perform differently with a different payload or application environment.
• The capstan diameter is a function of the recommended minimum bend radius of the cable, as the cable diameter increases, the capstan diameter increases. Because the drive ratio is the ratio of the drum and capstan diameters, drive ratios greater than 20:1 often require drums with diameters of several feet.
• The high drive efficiency means that the drive can be backdriven by an imbalanced payload if power is removed from the motors. Balancing the payload inertia about the Roto-Lok® drive axis is strongly recommended. Brakes may be required in applications where the payload cannot be adequately balanced.

The Importance of Stiffness, Torque Capacity, and Backlash

The three primary performance attributes of the Roto-Lok® Rotary Drive are its extremely high torsional stiffness, its high torque capacity, and its total freedom from backlash.

Why is Torsional Stiffness So Important?

To achieve extremely accurate rotary positioning, the coupling between a motor and the load must be so stiff that a miniscule position change of the motor armature produces a corresponding change in the position of the payload. If the drive transmission is not stiff enough this motion is lost in torsional windup and the payload does not move. Sagebrush has delivered hardware with one microradian (57 millionths of one degree) positioning capability and believes that it can achieve a 10 times better resolution if or when required.

Why is Torque Capacity Important?

Sagebrush has designed drives from 3 oz-in of output torque to more than one million foot pounds. There is no theoretical limit to the size loads that can be positioned with the Roto-Lok® drive. Drum diameter is a major factor in designing high-torque drives. Doubling the diameter of a Roto-Lok® drive allows for designing to an eightfold increase in torque capacity relative to the original diameter.

Why is Freedom from Backlash Important?

The drive's freedom from backlash makes it the preferred technology in applications where precision positioning is required. Backlash limits the accuracy in applications with frequent changes of direction. A positional error the size of the amount of backlash may also be present when external forces, such as gravity, wind loads or mechanical vibration are acting on the drive. Perhaps most importantly, in many current systems freedom from backlash greatly simplifies servo design, enabling the designer to obtain the desired degree of control.