Motion solutions turn to the rack

May 1, 2000
In an age of sophisticated electronic motion solutions, its easy to forget more traditional ways that work well. Case in point: rack-and-pinion gear assemblies that supply linear motion for drives, positioners, and measuring devices

When linear motion is required, rack-and-pinion assemblies may represent a simpler, more durable, and less costly solution for some applications. For example, lift trucks, power shovels, and other heavy machinery rely on rack-and-pinion gearing to convert rotary input to the straight-line movement of a reciprocating part. But rack-and-pinion devices aren’t limited to driving heavy loads. Other applications include rotary actuators for positioning, and precision measuring systems for machine tools.

The rack usually consists of a straight bar with teeth cut straight across it — essentially a spur gear that is laid out flat — and the mating pinion is a spur gear. Less common types include helical, worm, and herringbone.

In a typical arrangement, the rack-andpinion assembly converts rotary motion of a motor (connected to the pinion shaft) into linear travel of a driven component. Or the unit can work in reverse, converting linear motion into rotation.

Drive applications

Rack-and-pinion gearing is often used for linear drive applications in large machines that don’t require precise positioning. In such cases, the racks and pinions are typically made of low carbon steel. The rack is normally cold finished and the teeth are machined.

These rack-and-pinion units are economical and able to withstand adverse environments (dirt and moisture). But those with long racks may take too much space for some applications. Other ways to provide linear motion include

ball screws and hydraulic cylinders, both of which take less space, but cost more. Ball screws offer smooth, lowfriction motion. Hydraulic cylinders may experience leakage, thus, they may be better suited for mobile equipment than for factory applications.

Here are some representative drive applications as related to us by gear manufacturer Martin Sprocket & Gear Inc.:

Stacking glass. A glass manufacturing plant forms its product by pouring a continuous strip of molten glass on a bed of liquid platinum, which gives the glass a smooth finish. The glass cools as it moves along a conveyor, and is cut on the fly by a computer- controlled cutter into the required size. After cutting, a suction cup assembly picks up each piece of glass and stacks it. This lifting and stacking assembly moves with the aid of two gear racks connected to motor- driven pinions, Figure 1.

Heat treating. A large rack, approximately 4-in. wide by 4-in. thick and 60-ft long with 2-in. tooth-to-tooth spacing, transfers parts into a heat-treating furnace. This rack is attached to the floor between two rails, upon which rides a car that has a motor and 10-in. diam pinion on the underside. After the car is loaded with parts, the motorized pinion moves along the rack, driving the loaded car (70,000 to 80,000 lb) into the furnace where the parts are heat treated.

Car dumper. Curved racks operate a large rail car dumper at a paper manufacturing plant. At both ends of the 30-ft diam car dumper, a series of curved rack segments is mounted on the perimeter. Extending half way around the dumper at each end, the racks are 5-in. wide by 5-in. thick with 3-in. tooth-to-tooth spacing. They mesh with two 18-in. diam pinions spaced 60-ft apart near the bottom of the dumper, Figure 2. A large motor (about 100 hp) drives both pinions through a line shaft.

A rail car is pushed into the dumper and clamped to the rails. The pinions drive the racks through a 180-deg arc, turning the car upside down and dumping its load of wood chips. Then the pinions rotate the dumper back to the upright position, so the car can be unclamped and moved from the dumper.

Cutting Boards. A particle board plant replaced a series of sprockets and chains with racks and gears so that they could more efficiently cut lengths of particle board. The racks accelerate a flying table to the speed of the board as it moves along on a conveyor, then a saw cuts off an 8-ft long piece of board. After cutting, the racks decelerate and stop the table, then move the table back to its original position so it is ready for the next piece.

Locking mechanisms. Racks and gears also operate locking mechanisms for jail cells and gun safes. In these applications, a hand-operated wheel rotates one or more pinion gears, which cause the mating racks to move linearly so the high-strength locking pins enter their receptacles.

Rotary actuators

Air-operated rotary actuators perform functions such as positioning mechanical grippers and turning valves on and off.

One type, manufactured by Tol-OMatic Inc., rotates within a preset angular range to position a gripper in pickand- place operations. The operator sets the degree of rotation, usually between 0 and 180 deg, by adjusting stops that limit the rack stroke.

This actuator contains four basic parts: two pistons, a toothed rack, and a pinion, Figure 3. An adjustable guide block pushes the rack into mesh with the pinion to limit the amount of backlash. The racks and pinions are made from medium or high-carbon steel with machined (hobbed) teeth.

In a typical pick-and-place operation, the rotary actuator mounts on a linearslide assembly and supports a gripper, Figure 4. Through the combined motions of these three components, the machine picks up a part from one conveyor, turns it upside down, and places it on another conveyor. Once the gripper picks up a part, the actuator turns the gripper and part 180 deg (upside down from original position).

Here’s how the rotary actuator works: an operator or controller introduces pressurized air behind one piston, causing it to push the rack toward the other piston, which rotates the pinion (and gripper) 180 deg. Releasing the air and redirecting it behind the second piston causes the pinion to rotate back to its original position.

These units typically provide torque ranging up to 60 lb-in. at 150 psi. A similar device, called a vane actuator, achieves higher torque for such applications. For these units, external stops are available for setting the degree of rotation.

Another type of rotary actuator, manufactured by Conbraco Industries Inc., rotates 90 deg to operate a rotary valve. This unit contains two pistons with integrally cast racks, and a pinion. The racks and pinions are precision machined to ensure proper alignment of the pistons within the housing.

The Conbraco unit comes in single and double-acting models. In the double-acting version, the operator or controller introduces pressurized air between the pistons, which spreads them apart and rotates the pinion 90 deg to open or close a valve. Air between the pistons is then released, and air is introduced behind the pistons, which returns the actuator and valve to their original positions. In a single-acting model, when the air is removed, springs return the pistons to their original positions.

These units offer torque ranging from 25 lb-in. at 40 psi to 40,000 lb-in. at 120 psi.

Linear and angular measurement

In addition to drive and positioning applications, rack-and-pinion gearing can measure either linear or angular motion. These applications require precision racks and pinions.

With conventional straight-cut teeth, the rack and pinion must be precisely aligned to ensure accurate meshing. Because perfect alignment is impossible in practice, there is always some transmission error between the rack and pinion.

The Reliance Gear Co. in England avoids the need for precise alignment by using a thread-grinding process to generate helicoidal teeth on the rack, where the face of each tooth is curved. Helicoidal teeth tolerate up to 0.25-deg misalignment. They also produce an ellipsoid contact pattern when meshing with a straight-cut pinion. This prevents misalignment stresses in the rack teeth, and increases load capacity.

The racks are available in corrosionresistant, through-hardened 416 stainless steel.

Linear measurement. Linear measuring methods for machine tools include rack-and-pinion as well as electromagnetic and optoelectronic. The rack-andpinion systems, built to metric standards, are said to be cost effective, especially in lengths from 2.5 to 100 meters. On the other hand, size may be a limiting factor in some applications.

A typical measurement system contains an encoder or resolver, connected to the rack by a pinion, which detects linear motion of the rack, Figure 5. The system typically provides accuracies to 66 mm in 2,000 mm. The encoder or resolver has either a plain shaft for mounting the pinion, or a built-in pinion shaft for higher accuracy.

Rack-and-pinion assemblies are used in coordinate measuring systems at Mercedes, BMW, and Jaguar car manufacturers to check the three-dimensional accuracy of car bodies.

These systems can operate in harsh environments where cutting oils, moisture, and dirt are present. The racks are usually installed with the teeth pointing down, so that foreign matter falls out. Where further protection is needed, the rack is enclosed in a protective channel, Figure 6. Two flexible seals keep out contaminants while allowing movement of the encoder or resolver shaft along the rack.

Angular measurement. Although racks normally operate in the linear mode, certain types measure the angular position of rotary tables, such as those on vertical milling machines, Figure 7.

With an angular measuring system, rack segments are mounted on a cylindrical surface, rather than straight. A narrow face width (2.5 mm) gives these rack segments enough flexibility to conform to a cylinder of 570-mm diam or more. The rack circle consists of either 12 or 24 racks positioned every 15 or 30 deg around the circumference. The pinion, which meshes with the rack, drives an optical encoder or resolver to measure the rotation of the table with an accuracy to 610 arcsec.

This angular measurement technique even helps us track the stars — it measures the azimuth and elevation of the Jodrell Bank radio telescope in England.

For more on rack-and-pinion drives from Martin Sprocket & Gear, circle 311 on the reader service card.

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