Machine Drawing

82 Machine Drawing

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d:\N-Design\Des5-1.pm5 Seventh Print

Advances

Turn counter

clockwise

Left hand

Advances

Turn clockwise

Right hand

Fig. 5.5 Right hand and left hand threads

5.7.1 Coupler-nut

A coupler-nut or turnbuckle is an example of a machine element, in which both right hand and

left hand thread forms are used. The length of a tie rod, may be adjusted by this device. Referring

the Fig. 5.6a ; out of the two rods operating inside the nut (a long double nut), one will have a

right hand thread at its end and the other, a left hand one. The nut is usually hexagonal at its

outer surface, with a clearance provided at the centre. By turning the nut, the two rods in it may

move either closer together, or away from each other. Figure 5.6b shows a coupler used for railway

coaches. They are also used for fixing guy wires, etc.

60

f

55

f

35

12

95

60 170 60

f

35

12

95

(a) Adjustable joint for round rods

f 15

KNURLED SQ THD × 8 (LH)

f

50

f

36

120 70 120

(b) Coupler for railway coaches

SQ THD × 8(RH)

f

40

Fig. 5.6

5.8 Representation of Threads

The true projection of a threaded portion of a part consists of a series of helices and it takes

considerable time to draw them. Hence it is the usual practice to follow some conventional methods

to represent screw threads. Figure 5.1 shows the true projection of a screw thread, whereas the

conventional representation of external and internal threads as recommended by BIS is shown in

Fig. 5.7.