To transform a rotational movement into linear movement with high forces mostly spindles or racks are used.
For screw jacks the pitch and the number of threads are the main variables and for racks it is the modulus and the pinion's tooth number.
Due to „Niemann“: η=tanα/tan(α+ς) screw jacks have bad efficiency because of the sliding between nut and spindle.
Causal for this is the high sliding ratio in the movement (cf. Niemann: „Maschinenelemente“ Band 1, Springer Verlag, 2. edition 1981, p. 203).
With toothed gears the revolving movement, the rollover, outweighs the sliding. The efficiency ratio is, conferred to „Linke“, definded over the loss ratio of the cogging: cogging loss = friction coefficient * cogging loss factor = Vz = µ * Hv. tooth efficiency = ηz = 1 – Vz (cf. Linke: „Stirnradverzahnung“ Hanser Verlag, 2. edition, 2010, pp. 387).