Power screws

APPLICATION OF POWER SCREWS

The function of a power screw is to provide a means for obtaining a large mechanical advantage and at the same time transmitting power by converting angular, into linear motion. Common applications include lifting jacks, presses, vices, and lead screws for lathe machines. Figure 1.1shows the application in a lifting jack, while Figure 1.2 shows the same concept when used for a press.

SCREW JACK APPLICATION

SCREW PRESS APPLICATION

THREAD FORMS FOR POWER SCREWS

Power screws use either square, or trapezoidal thread forms. Two types of trapezoidal thread forms are the ACMEthread standard, used widely in the English speaking countries, and based on the inches units, and the Metric trapezoidalstandard, originating in Europe, and now adopted by the International Standards Organisation (ISO).
Figure 1.3 shows the three geometric profiles of the three thread forms used for power screws.


The (ISO) Metric trapezoidal thread form standard includes specifications that relate screw shaft diameter to pitch, as shown in Table 1.1. For the square and ACME thread form standards, only the geometric profile of the thread form is specified, and the designer is left to chose the size of thread for each screw shaft diameter. This does not pose any serious problem because each power screw application is often a special case.

Table 1.1: (ISO) Metric Trapezoidal screw thread standards-Diameter and Pitch specifications

Nominal (Major Exernal) Diameter	Pitch			Pitch Diameter	Major Internal Diameter D	Minor Diameter
	Coarse	Medium	Fine			External	Internal
8		1.5		7.25	8.30	6.20	6.50
10		2	1.5	9.00	10.50	7.50	8.00
12		3	2	10.50	12.50	8.50	9.00
16		4	2	14.00	16.50	11.50	12.00
20		4	2	18.00	20.50	15.50	16.00
24	8	5	3	21.50	24.50	18.50	19.00
28	8	5	3	25.50	28.50	22.50	23.00
32	10	6	3	29.00	33.00	25.00	26.00
36	10	6	3	33.00	37.00	29.00	30.00
40	10	7	3	36.50	41.00	32.00	33.00
44	12	7	3	40.50	45.00	36.00	37.00
48	12	8	3	44.00	49.00	39.00	40.00
52	12	8	3	48.00	53.00	43.00	44.00
60	14	9	3	55.50	61.00	50.00	51.00

Figure 1.4 illustrates the dimensions of the (ISO) Metric Trapezoidal thread standard shown in Table 1.1.

MECHANICS OF POWER SCREW (SQUARE THREADED)

Figure 1.5 shows the geometry and dimensions of a square threaded power screw, with a single start thread.

The power screw carries an axial load F, and this is to be raised or lowered by applying a turning moment or torque on the screw shaft. The screw and nut machine then coverts the torque on the screw shaft, into the desired axial load. This is the typical situation in the screw jack, and the screw press concepts shown at Figures 1.1 and 1.2.