Screw threads may be right hand or left hand, depending on the direction of the helix. A right hand thread is one which advances into the nut, when turned in a clockwise direction and a left hand thread is one which advances into the nut when turned in a counter clockwise direction. An abbreviation LH is used to indicate a left hand thread. Unless otherwise stated, a thread should be considered as a right hand one. Right hand and left hand threads figure given below illustrates both right and left hand thread forms.
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 “Adjustable joint for round rods image” 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. Coupler for railway coaches figure shows a coupler used for railway coaches. They are also used for fixing guy wires, etc.