(input shaft vs output shaft) is not reversible when using large reduction ratios. This is due to the greater friction involved between the worm and worm wheel, and is especially prevalent when a single-start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. If a multi-start worm (multiple spirals) is used, then the ratio reduces accordingly, and the braking effect of a worm and worm wheel may need to be discounted, as the wheel may be able to drive the worm. Worm drive configurations in which the wheel cannot drive the worm are called self-locking. Whether a worm drive is self-locking depends on the lead angle, the pressure angle, and the coefficient of friction. Applications In early 20th century automobiles prior to the introduction of power steering, the effect of a flat or blowout on one of the front wheels tended to pull the steering mechanism toward the side with the flat tire. The use of a worm drive reduced this effect. Further worm drive development led to recirculating ball bearings to reduce frictional forces, which transmitted some steering force to the wheel. This aids vehicle control, and reduces wear that could cause difficulties in steering precisely. Worm drives are a compact means of substantially decreasing speed and increasing torque. Small electric motors are generally high-speed and low-torque; the addition of a worm drive increases the range of applications that it may be suitable for, especially when the worm drive's compactness is considered. Worm drives are used in presses, rolling mills, conveying engineering, mining industry machines, on rudders, and circular saws. In addition, milling heads and rotary tables are positioned using high-precision duplex worm drives with adjustable backlash. Worm drives are used on many lift/elevator and escalator-drive applications, due to their compact size and their non-reversibility. In the era of sailing ships, the introduction of a worm drive to control the rudder was a significant advance. Prior to its introduction, a rope drum drive controlled the rudder. Rough seas could apply substantial force to the rudder, often requiring several men to steer the vessel—some drives had two large-diameter wheels so that up to four crewmen could operate the rudder. Worm drives have been used in a few automotive rear-axle final drives (though not the differential itself). They took advantage of the location of the worm