Friction —— good and bad
Ballscrews operate with relatively little friction, making them very efficient — 95% or better. Even more efficient are ballscrews employing nonuniform balls. "Alternating ball sizes within the ball nut assembly greatly increases efficiency," says Andy O'Connell of Rockford Ball Screw Co., Rockford, Ill. "The technique reduces friction by changing the rotational direction between succesive balls."

There is one caveat, however. Backdriving occurs when the lead angle tangent exceeds the coefficient of friction at the thread interface. Because ballscrew assemblies have a friction coefficient very close to zero, they tend to backdrive under almost any load in vertical applications. In other words, less resistance to motion means they're pulled by gravity out of position. Backdriving on a ballscrew can be prevented with a brake on the screw or motor — useful if the additional cost of a brake is worth the efficiency gained with ballscrews.

More often, however, leadscrews are better suited for vertical applications. Unlike ballscrews, leadscrews have distinct friction, which is also variable by design. "Think of a block and a sphere on a flat surface," says Tom Solon of Kerk Motion Products Inc., Hollis, N.H. "Any inclination causes spheres to roll, but blocks remain stationary until the surface is inclined such that the forces parallel to the surface exceed the friction forces," Solon explains. In this case, the lower efficiency of leadscrews actually prevents them from backdriving — so they can function as self-locking mechanisms.

Gear up with synthetics
Lubricants can increase the efficiency of just about any mechanical motion component. Synthetic lubricant in particular, though more expensive, is often justified by lower energy consumption. Darren Lesinski, a technical service and OEM compressor-fluid development manager at Anderol Inc., East Hanover, N.J., explains: "Synthetics can be customized to environments, and they typically maintain their original properties longer — sometimes reducing energy consumption by 15% — 10% more than with mineral varieties. On the other hand, mineral-based lubricants are subject to things like fluid churning, friction wear, and an increased need for relubrication."

Synthetic polyglycol-based lubricants (PAGs) are especially beneficial for gears. "Gears operating with PAGs experience less friction because of the lubricant's engineered viscosity and high molecular weight," says Lesinski. Synthetics also reduce temperature and change-out frequency while preventing rust and corrosion. "The cling properties of nonasphaltic synthetic lubricants help form a protective coating that actually cools equipment," Lesinski explains.

Lubricants with an appropriate balance of base oil and additive reduce power consumption most. Some include antioxidants or extreme pressure additives for components that must withstand heavy loads. Outdoor machinery in colder climates often run at -25° to -50°C or lower. The low viscosity and pour point of synthetics are especially beneficial to such machinery because it improves pumpability. "On cylindrical gears, PAG and polyalphaolefin oils in particular exhibit less churning losses at low temperatures and less gear erosion, reducing costly maintenance," says Lesinski. Other benefits include reduced start-up torque and lower current draw.