Practical Automation: Using mechanical methods to generate linear motion

Mechanical rotary-to-linear motion conversion drives can move and position loads with varying degrees of precision, but must be guided in their lineal motion to be effective in practical automation applications.

In the previous issue of Machinery & Equipment MRO (Dec.1999, p. 23), I talked about linear motion as generated by hydraulic or pneumatic cylinders. Linear motion can also be generated by the mechanical conversion of rotary motion.

These linear devices are generally used for moving and positioning loads with varying degrees of precision. The electric motors used to drive these devices are mostly DC, stepping or servo types with some method of control loop feedback. Cams and cranks, however are simpler linear mechanisms not requiring the same degree of control sophistication.

Cams

The generation of mechanical motion through the use of cams seems to be becoming a lost art. Machines using a central cam system to drive all motion have long since been replaced by the innovative use of other, more accurate and productive motion generators.

Cams, however, are still frequently used as a convenient and economical way to drive position sensors such as limit switches and valves. Using rotary cam followers generally reduces the friction and wear developed from two sliding surfaces.

Rotary indexing devices using barrel cams to achieve motion control between stations are still considered to be accurate and reliable components favoured in automated machinery.

Cranks

The conversion of angular motion to lineal motion through the use of cranks creates some very interesting and useful side effects. The reciprocating lineal action created with a crank has the added benefit of delivering its maximum thrust at the end of its stroke, making it ideally suited for punch presses or toggle-type clamps.

The sinusoidal acceleration and deceleration of motion generated by the lineally guided end of the connecting rod provides a very smooth and inexpensive method of dealing with a large inertial load–as found in large transfer lines or indexing machines. The over-centre locking action caused by stopping the cranking action against a positive stop just past the bottom dead centre of the crank allows the toggle clamp, a crank-activated device, to hold its clamping position without expending any energy.

Lead screw assemblies

Lead screws using an acme thread mated with a plastic or bronze solid nut that slides along the threads of the screw–much like an ordinary nut and bolt–are beneficial when the ability to hold a load in a power-off situation is required. Since there are no rolling elements between the nut and the lead screw, they yield only 30% to 50% of the motor’s energy to driving the load. The remaining energy is lost to friction and dissipated as heat. This heat generation limits the dynamic duty cycle to less than 50%.

The power-off load-holding capability is directly related to the coefficient of friction between the nut and screw and the helix angle of the screw thread. The acme lead screw is well suited to low- to medium-thrust applications requiring low speeds and low duty cycles.

Ball screws

Ball screw assemblies are widely used for lineal motion because they are capable of converting up to 90% of a motor’s torque to thrust. The ball nut uses one or more circuits of recirculating steel balls running in matching nut and ball screw threads.

The low frictional operation of these assemblies results in some backlash which is easily removed by pre-loading one recirculating nut assembly against another. The free-running nature of the ball screw design does not allow backloading in a power-off state at all.

The maximum speed of the assembly is limited by the harmonic instability (whipping action) of the unsupported length of the screw at a higher rpm. The column strength of the screw limits the maximum thrust it can exert.

Roller screws

Roller screws are similar in operation to ball screws but have a much higher thrust capability, about 15 times the life expectancy, and can operate up to five times as fast (5,000 rpm). Unfortunately, they cost a lot more too.

The multiple threaded helical rollers used to transfer the torque to the screw are assembled in planetary fashion around the threaded shaft and held in place by fixed journals at the ends of the annular nut cage. This non-recirculating roller construction is particularly well suited to applications involving the highest loads and the highest speeds. Recirculating rollers are used where precise positioning, high loads and a high level of rigidity are required.

Timing belt positioners

Belt drive systems offer many of the benefits of ball screw and roller screw devices, yet have fewer moving parts, and do not have the critical speed limits of lead screw-driven systems. They generally provide more linear motion from the same motor movement, resulting in higher travel speeds with minimal component wear.

In contrast, this design results in lower repeatability and accuracy. Thrust capability is also smaller compared to screw drive systems due to the tensile strength limitation of the transport belt. Timing belt systems are a good solution for applications requiring high speeds, low thrusts, high efficiencies, high duty cycles and lower positional accuracies.

All of the devices mentioned here are rotary-to-linear conversion drives. Most of them must be guided in their lineal motion to be effective. These guiding devices may be ball slides, roller slides, dovetail or hardened steel slides, linear bearings, or other articulated guiding devices–each to be a subject of a subsequent article in this series (next up will be linear induction motors and solenoids).

Ted Grove, corporate training manager for Wainbee Limited of Mississauga, Ont., is an experienced fluid power trainer. Visit www.mromagazine.com on the Internet and click on the Past Issues button to view previous Practical Automation columns from the September, November and December 1999 issues of Machinery & Equipment MRO.