Forms of Threads:-

1. Square Threads:- A Square threads, shown in figure are used for transmission of power in either direction.

Advantages of Square Threads:-

1. Square Threads have maximum efficiency of all threads form.
2. They exert minimum radial or bursting pressure on nut.
3. They can transmit power in either direction.

Limitations of Square Threads:-

1. Strength of the square threads in lowest of all the thread forms.
2. These Threads cannot be used conveniently with split nut because:
   (a) wear compensation is not possible and
   (b) engagement and disengagement is difficult.

Applications of Square Threads:- The Square Threads are used in screw jacks, mechanical processes and damping devices.

2. Acme Threads:- The Acne threads shown in figure have thread angle equal to $29\degree$.

Advantages of Acme Threads:-

1. Acme Threads permit the use of split nut which can compensate the wear.
2. Acme Threads are stronger than the square threads in shear because of the larger cross-section at the root.
3. Acme Threads can transmit power in either direction.

Limitations of Acne Threads-:
(i) Because of slope gives to the sides, the efficiency of the acme threads is lower than that of square threads.
(ii) The slope on the sides introduces some bursting pressure on the nut.

Applications of acme threads:- Acme Threads are used for lead screws of machine tools, bench vices, etc.

3. Trapezoidal Threads:- The Trapezoidal Threads, as shown in figure are similar to acme threads , except the thread angle which is $30\degree$ in trapezoidal threads.

Advantages of Trapezoidal:-
(i) It is economical to manufacture.
(ii)The strength is more than square threads.

Limitations:-
(i) The efficiency is less than that of square threads .
(ii) Here sides thrust exist on the nut so the performance is affected.

4. Buttress Threads:-
Buttress Threads as shown in figure are used where force or power is required to be transmitted only in one direction.

Advantages of Buttress Threads:-
(i) Buttress Threads are stronger in shear from the any other power threads because of the largest cross section at the root.
(ii) The Buttress threads combine the high efficiency of square threads and high strengths of V- threads.

Limitations of Buttress Threads:-
(i) Buttress threads are used to transmit power in only one direction.
(ii) These Threads are difficult to manufacture.

Applications of Buttress Threads:-
These threads are used in vices and screw jacks where force is to be applied only in one direction .