Power Screw Theory

Power Screw - 


                   Power screw is a mechanical device which is used to convert rotary (turning) motion into to the translational (linear) motion. It is used as a linkage in a machine. Linkage is a assembly or a network of rigid linkages used to manage forces and moments. It uses helical motion of screw to transmit the power. 


Main applications of power screw 
1) used to raise the load       eg. Screw Jack 
2) to obtain accurate motion        e.g.Lead Screw of lathe 
3) to load a specimen on UTM [Universal Testing Machine] 
4) to clamp a workpiece       e.g.vice 

Advantages of power screw :-

1. It has large load carrying capacity
2.  It has small dimensions and easy to design n manufacture 
3. Highly accurate translational motion 
4. Minimal number of parts engaged in this device 

Disadvantages:-

1.  Poor efficiency due to two large frictional losses
2. Due to high friction; rate of wearing is high 

notes are made of soft material then screws so that affect all the failure takes place not fails and not the screw screw is costlier and difficult to replace 

Types of power screw [on basis of threads or forms of threads] :-

          Power Screws are classified on the basis of geometry of their threads. 
{Note:-  'Power screws' and 'Fasteners' both have threads.  V shaped threads are commonly used in fasteners as their function is to join two objects 'non permanently'.  V threads provide friction to minimise the chances of loosening.   V threads are exact opposite of threads used in power screws. Threads in power screw provide low friction between nut and bolt so as to transmit power. 
Types of threads commonly used in power screw :-
 1) Square 
 2) ACME 
 3) Trapezoidal 
 4) Buttress thread 


1] Square Thread:- 
        

• They have much higher efficiency and lowest friction. 
• They are hard to manufacture and difficult in machining.
• Due to ZERO degree thread angle, there is no radial pressure or bursting pressure on Nut. This increases nut life.
•  used in high load applications such as screw Jack, clamping device.
•  It has less thread thickness than other types. 
• It's not possible to use split type nut in this case [Split nut is that which can split into two equal halves so that it can be positioned on a shaft or belt] 

2] ACME Thread :- 

• It has 29 degree thread angle 
• Strength of screw depends upon thread thickness of core diameter. ACME and trapezoidal threads are stronger than square threads due to more thread thickness 
• Due to presence of thread angle, nut is subjected to radial or bursting pressure and hence efficiency is less than square threads.
• Split type nuts can be used here 
• These threads are used in lead screw and other power transmission devices.

3] Trapezoidal thread 

• ACME  and trapezoidal screws are mostly similar except the thread angle.
• Trapezoidal thread angle is 30 degree while ACME thread angle is 29 degree.
• Uses same as ACME. 

4] Buttress thread

•  They are designed such that force or load is applied only in one direction.
•  Strongest thread as it has largest thread thickness
•  Easy to manufacture and comparable efficiency with square screws.
•  Used in light Jack screw and vices.

Parts of Power Screw:- 
   ➡ screw 
   ➡ nut 
   ➡ a part which hold either nut or bolt in respective place.

 Motion in two way:-
1) Screw moves in axial direction and nut kept stationary.
    e.g. screw jack and vice 
2) Nut moves in axial direction and screw kept stationary.
    eg .  Lead screw of lathe machine 

This was basics about power screw.


Screw Jack:- 

•  Screw Jack is a special case of power screw which raises or lowers the load by applying a small force in the horizontal plane.
•  Screw thread is nothing but helical grows which are cut by either left or right hand mainly used to lift heavy weights.
•  Screws which are tightened in clockwise direction term as right hand thread, screws with left hand threads are used exceptionally.

Fundamentals of Mechanical Engineering [Part 1]

Lets begin with the basic question😄.......

What is Machine ?

• A machine is a device, consisting of various elements arranged together, so as to perform the prescribed task to satisfy the human needs. 
• Machine is capable of doing some useful work. It converts and transfers different forms of energy.
• Modern machines are complex systems that consist of structural elements, mechanisms and control components.
• INPUT ⟹ MACHINE ⟹  OUTPUT
• Ex. of Machine- I.C. Engine, Turbines, Compressors, C-clamp , Screw- Jack,etc. 

Machine Elements:-    

• Machine elements are basic mechanical parts 
• Machine elements are assembled together to form what we call as complete machine.
• Its function may be of holding the components together, to transmit power or to give supports.
• Ex. Elements of Screw Jack:-   Screw, Nut, Body, Handle    

  Some important Machine Elements:-

         1] Shaft :- To transmit the power

         2] Key :- To transmit the power from the shaft to other rotating elements

         3] Couplings :- To connect two shafts

         4] Bearings :- To support the shaft

         5] Nut and Bolt:- To connect two parts

        Lets see one by one...

  SHAFT:-

•  It’s rotating machine of circular cross section which is used to transfer power.
• Transmission of power from one shaft to another takes place by mounting ‘gears, pulleys, crank, sprockets etc. on it. 
• These members are mounted on the shaft by means of keys or splines.
• They are subjected to torque, bending moment, axial force. 
• Sometimes, hollow shafts are used to reduce the weight.

Types of Shafts:-

         A.    Transmission Shafts

         B.    Machine Shafts

A.    Transmission Shafts :- Shaft which is used to transmit the power from source to the machine absorbing power is called Transmission Shaft. Transmission shaft supports ‘ power transmission elements’ like pulleys, belts, sprockets, chains, gears, clutches.

      -  Examples of transmission shaft:

                Line Shaft and Counter Shaft 

B.     Machine Shafts:-   

It is a short rotating shaft which forms an integral part of the machine itself.

Ex.

Spindle Shaft :- A spindle is a rotating shaft with a fixture for holding a tool or a workpiece.

Crankshaft:-

      A crankshaft is a rotating shaft which converts reciprocating motion of the pistons into rotational motion.

Crankshafts are commonly used in internal combustion engines and consist of a series of cranks and crankpins to which the connecting rods are attached.

Axle:-

1. An axle looks a like shaft but it’s a non-rotating machine element which is used to support rotating elements.

2. The main purpose of axles is to transfer power to the rotating elements and to support the rotating machine elements.

3. Axle is subjected to bending moment and axial force but not torque as it’s not rotating

       

     Ex. Axle of car etc.

Car rear axle

Key:-

• Key is a machine element used on the shafts to secure rotating elements by inserting it between shaft and hub of element and prevents relative motion between them. Rotating elements like- gears, pulleys, sprockets.
• The secondary function of the keys is to prevent relative movements in the axial direction of the shaft.

•  Key is also used for transmitting the torque from shaft to rotating element or vice-versa.

• It’s having lower strength than shaft material to ensure that if at all there is failure the key should fail instead of shaft.

• Keyway is a slot or pocket in a shaft and hub of rotating element to insert a key.

Types of Keys:-

     A. Sunk keys :- Sunk Key is that kind of key that is mount half a side in hub of rotating element and half a side in shaft keyway. Most widely used keys are sunk keys.

Woodruff Key

Reactangular Key

Parallel Key

Gib- head key

Square Key

                                             

B.Saddle Keys:- 

• It’s tapered key fits only in hub of rotating element.
• Saddled key doesn’t sink inside the shaft , hence it’s not a sunk key.

They have low torque transmitting power. As it is outside the shaft, it doesn’t weaken the shaft.

    

          C.Tangent Keys:- 

• These are used in large heavy weight shafts.
•   It transmits the torque only in one direction by means of compressive force alone. Two tangent keys are separated by  90 degree.       

 D. Feather Keys:- Keys which allows some relative axial motion between hub and shaft but definitely prevents  relative rotational motion between them.

 

Feather key is used when the hub is required to slide along the shaft.

Feather Key which allows some axial motion.

E. Splines:-

• The splines are keys made integral with the shaft
• It prevents relative rotational motion but permits relative axial motion. 
•  Splined shaft is used when forces to be transmitted are large in magnitude. They are used in automobile gear boxes and machine tool gear boxes.                                                                                                                                 

         

Spline and Gear Shaft

                                       to be continued.............