power screw and threads

POWER SCREW

             A mechanical device meant for converting rotary motion into translational motion for transmitting power. Main applications of power screws are:

(i)         Raise the load, eg: Screw Jack

(ii)        Accurate motion in machine operation

(iii)       To clamp work piece

(iv)       To load a specimen

             The main advantage of power screws are their large load carrying capacity with small overall dimensions.  Power screws are simple to design, easy to manufacture and give smooth and noiseless service.

FORMS OF THREADS:

             There are four types of threads used in power screws: They are acme, I.S.O. metric, trapezoidal, and buttress.

Selection guide lines:

1.          The efficiency of square thread is more than that of other types of threads.

2.          Square threads are difficult to manufacture.

3.          The strength of a screw depends upon the thread thickness at the core diameter.

4.          The wear of the thread surface becomes a serious problem in application like the lead screw of the lathe.

5.          Buttress thread can transmit power and motion only in one direction.  While square and trapezoidal threads can transmit force and motion in both direction.

FORCE ANALYSIS

             The major dimensions of the power screws are shown in figure.

             d – Nominal or outer dia (mm)

             d_c – core or inner dia (mm)

             d_m – mean diameter(mm)

Selection guide lines:

             When square threads are used for the screw, it can be treated as an inclined plane wrapped helically round a cylinder.  The helix angle a of the thread is given by:

Tan a  is inversely proportional  πdm

             W, is the load which is raised or lowered by rotating the screw by means of an imaginary force P acting at the mean radius.  There are two different cases depending upon the load being raised or lowered.

References

1. Mechanical Engg. Design                               –          Joseph Shigley
2. Machine Design                                              –          Mubeen
3. Machine Design                                              –          Black
4. Machine Design                                              –          R. K. Jain
5. Machine Design an integral approach             –          Norton, Pearson
6. Machine Design data hand book                                –          Lingayah Vol I.
7. Elements of Machine Design                         –          Pandya & Shah