# Screw surfaces - Engineering graphics

## Screw surfaces

Screw surfaces are very widely used in engineering for forming parts for various purposes: fasteners - screws, bolts, studs, screws and associated parts; special screws that convert rotational motion into translational motion, etc.

The helical surface is formed by the motion of a rectilinear generatrix along two guides, one of which is a helical line, the other is the axis of the helix that the generator intersects at a constant angle.

Direct helical surface. For a straight screw surface, the angle between the generatrix and the axis is 90 °. This is a helical conoid or straight helicoid. A drawing of a straight helical surface is shown in Fig. 8.8. Moving in the direction, as indicated by the arrow on the horizontal projection, the segment AB moves along the axis upwards and the image -

Fig. 8.8

Fig. 8.9

Have the right screw surface. Projections are conditionally shown as two lines (they are removed from the viewer).

In the section of the straight helical surface (Fig. 8.9), planes that are perpendicular to the axis or passing through the axis produce straight line segments. Using them, you can build points on the helical surface. Thus, in Fig. 8.9 on the horizontal projection A ' of the point A its frontal projection A on the front projection of the generator 1 & 2; in the cutting plane β (β '). From the frontal projection B of the point B its horizontal projection B ' is constructed on the horizontal projection of the generator 3'4' in the cutting plane

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A direct screw surface is used in special screws to convert rotational motion into translational motion into

Fig. 8.10

precise screwdrives or screwdrives with large axial forces, for example in presses.

Slanting helical surface. If the angle between the generatrix and the axis is not 90 ° at the helical surface, then it is called an oblique screw surface. The oblique helical surface is shown in Fig. 8.10, a. The projections of the segment AO - of the generator are shown in a series of successive positions from the first to the thirteenth. The point A of the generator moves along the helix. The corresponding positions of the projections of the point O are marked on the axis, being guided by the fact that the projection of the segment AO on the rotation axis is constant in magnitude ( l ) •

The construction of the cross-section of an oblique screw surface by a plane perpendicular to the axis is shown in Fig. 8.10, b. Such a plane intersects the surface along the curve of the line - the Archimedes spiral. Section construction is performed on the points C1, C2, C3, C4, C5 of the intersection of the secant plane γ (γ ) with the generatrix of the helical surface in a series of consecutive positions, as well as with a screw line at the point

For the construction of horizontal projections then a check of the Archimedes' spiral carries out the horizontal projections of the generatrix of the helical surface in a number of arbitrary positions O'T, 0'2 ', 0'3', 0'Ai, 0'5 '. Frontal projections of 1 ", 2", 3 ", Ai', 5" points are marked in the projection connection on the frontal projection of the helix. Through them, taking into account that the magnitude of the projection of the generatrix on the axis of the helical surface is constant (its value/is marked in the drawing for the construction of the point O1 "), the front projections of the generators are constructed. At the intersection of these front projections with the front track γ of the secant plane, the front projections are marked and horizontal projections are constructed on them in the projection relationship of the desired points on the corresponding horizontal projections of the generator. Through the points constructed, a smooth curve is drawn.

If a frontal projection of an arbitrary point M of the helical surface is given, then its horizontal projection is constructed using a section plane perpendicular to the axis, as discussed in Fig. 8.10, b. If a horizontal projection of the point (M ') is given, then the horizontal projection O'K' of the generator is drawn through it, the frontal projection from the projection K and the value of/- the projection of the generator on the axis of the helical surface. On the constructed projection the generator is marked with the front projection M " of the point M.

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