Power Screws MCQ Quiz - Objective Question with Answer for Power Screws - Download Free PDF

Concept:

The maximum efficiency of screw jack is when helix angle α is

\(\alpha = 45^\circ- \frac{\phi }{2}\)

Note:

If the friction angle is greater than the helix angle, the torque required to lower the load will positive, indicating that an effort is applied to lower the load. Such a screw is known as a self-locking screw.

Additional Information

Screw jack

• A screw jack is a portable device consisting of a screw mechanism used to raise or lower the load. There are two types of jack Hydraulic and mechanical
• A Hydraulic jack consists of a cylinder and piston mechanism. The movement of piston rod is used to raise or lower the load.
• Mechanical jack can be either hand operated or power driven

The maximum efficiency of screw jack is:

\(\eta = \frac{{1 - sin\phi }}{{1 + sin\phi }} \)

Explanation:

Effect of Adding a Locking Feature to a Threaded Fastener

• In mechanical assemblies, maintaining the integrity and stability of fasteners is crucial to ensure the safety and reliability of the structure.
• One common challenge is the loosening of fasteners due to vibrations, which can compromise the assembly.
• To address this issue, various locking features can be added to threaded fasteners. One such feature is the nylon insert, also known as a nylon locknut.
• A nylon insert is a polymer ring placed within the threads of a nut.
• When the nut is tightened onto a bolt or screw, the threads cut into the nylon, creating frictional resistance.
• This resistance prevents the nut from backing off, even under conditions of vibration and dynamic loads.

Primary Effect:

The primary effect of adding a locking feature, such as a nylon insert, to a threaded fastener is:

Option 1: Prevents the fastener from loosening due to vibration.

This is the correct answer. The nylon insert creates a frictional force that resists rotation, effectively preventing the fastener from loosening when subjected to vibrations and dynamic forces. This characteristic is particularly valuable in applications where vibration is a significant concern, such as in automotive, aerospace, and machinery components.

Advantages:

• Enhanced reliability of mechanical assemblies by preventing loosening of fasteners.
• Cost-effective solution for improving the stability of threaded connections.
• Easy to implement without the need for additional tools or complex procedures.

Disadvantages:

• May not be suitable for high-temperature applications where the nylon can degrade or melt.
• Potential for wear and degradation of the nylon insert over time, especially in high-vibration environments.

Applications:

• Automotive industry: Used in various components to ensure fasteners remain secure under the vibrations of a moving vehicle.
• Aerospace industry: Critical in maintaining the integrity of assemblies in aircraft subjected to dynamic forces and vibrations.
• Machinery and equipment: Commonly used in industrial machinery where vibrations are prevalent.

Concept:

Screw threads are frequently used in jacks, presses, and other machines.

Screw thread acts as an inclined plane of angle if inclination equal to the helix angle α as shown in the figure.

The helix angle of the thread is α, where, tanα = \(\frac{np}{2\pi r}\)

where, np = lead of screw, r = mean radius of a screw.

The helix angle 'α' can be greater than the friction angle 'ϕ'. where ϕ = tan-1 μ  where μ = coefficient of static friction.

Let us consider, α > ϕ,

In this case, once the load on the screw jack is lifted, then after removal of the effort from the handle, the screw threads will roll down and load will be lowered.

The screw jack will become reversible, which is undesirable for a screw jack.

Therefore, for a screw jack, helix angle α, of threads should always be less than friction angle ϕ.    

Explanation:

Screw jack

• A screw jack is a portable device consisting of a screw mechanism used to raise or lower the load. There are two types of jack Hydraulic and mechanical
• A Hydraulic jack consists of a cylinder and piston mechanism. The movement of piston rod is used to raise or lower the load.
• Mechanical jack can be either hand-operated or power-driven.   

The maximum efficiency of the screw jack is:

\(\eta = \frac{{1 - sin\phi }}{{1 + sin\phi }} \)

The maximum efficiency of screw jack is when helix angle α is

\(\alpha = 45^\circ- \frac{\phi }{2}\)

Additional Information

The screw used in screw jack:

• The screw jack is intermittently used device and wear of thread is not an important consideration. Therefore instead of trapezoidal or V threads, the screw is provided with square threads.
• Square threads have higher efficiency and provision can be made for self-locking arrangement.
• When the condition of self-locking is fulfilled, the load itself will not turn the screw and descend down, unless the handle is rotated in reverse direction with some effort

Explanation:

Screws

The screws are used to convert rotary motion into translatory motion.

Following are the types of screw threads mostly used for screw:

Square thread

• A square thread is adapted for the transmission of power in either direction.
• This thread results in maximum efficiency and minimum radial or bursting pressure on the nut.
• It is difficult to cut with tap and dies.
• It is usually cut on a lathe with a single point tool and it cannot be easily compensated for wear.
• The square threads are employed in screw jacks, presses and clamping devices.

Acme or trapezoidal thread

• An acme thread is a modification of square thread.
• The slight slope given to its sides lowers the efficiency slightly than square thread.
• It also introduced some bursting pressure on the nut but increases its area in shear.
• An acme thread may be cut by means of dies and hence it is more easily manufactured than square thread.
• It is used where a split nut is required and where provision is made to take up wear as in the lead screw of a lathe.

Buttress thread

• A buttress thread is used when large forces act along the screw axis in one direction only.
• This thread combines the higher efficiency of square thread and the ease of cutting and adaptability to a split nut of an acme thread.
• It is stronger than other threads because of greater thickness at the base of the thread.
• The buttress thread has limited use for power transmission.
• It is employed as a thread for light jack screws and vices.

Knuckle thread

• A knuckle thread is another modification of square thread.
• The sharp corners of square thread are rounded to make the crests and roots semicircular.
• The crests and the roots are tangent to each other.
• The profile is difficult to machine and hence these threads are usually manufactured by rolling or casting.
• Due to its unique shape, these threads are less subjected to wear and suitable for rough usage.
• Knuckle threads are provided on the coupler of the railway carriage, hydrants, glass bottles, electric bulbs, etc.

Explanation:

Square and trapezoidal threads are most commonly used threads.

Square thread: In this thread the flanks are perpendicular to the axis of the thread. This is used for transmitting motion or power. E.g. Fly presses, Screw jack, vice handles, cross-slide and compound slide etc.

Trapezoidal threads: These threads have a profile which is neither square nor V thread form and have a form of trapezoid. Different forms of trapezoidal threads are ACME thread, Buttress thread, Saw-tooth thread, Worm thread

• ACME thread: It has an included angle of 29°. These are used in lathe lead screws.
• Buttress Thread: In this thread, one flank is perpendicular to the axis of the thread and other flank is at 45°. These threads are used in power press, carpentry vices, gun breeches, ratchets etc.
• Knuckle thread: The shape of the knuckle thread is not trapezoid, but it has a rounded shape. It has limited applications. It is used for valve spindles, railway carriage coupling, hose connections etc.

Concept:

The maximum efficiency of screw jack is when helix angle α is

\(\alpha = 45^\circ- \frac{\phi }{2}\)

Note:

If the friction angle is greater than the helix angle, the torque required to lower the load will positive, indicating that an effort is applied to lower the load. Such a screw is known as a self-locking screw.

Additional Information

Screw jack

• A screw jack is a portable device consisting of a screw mechanism used to raise or lower the load. There are two types of jack Hydraulic and mechanical
• A Hydraulic jack consists of a cylinder and piston mechanism. The movement of piston rod is used to raise or lower the load.
• Mechanical jack can be either hand operated or power driven

The maximum efficiency of screw jack is:

\(\eta = \frac{{1 - sin\phi }}{{1 + sin\phi }} \)

Explanation:

Square thread: In this thread, the flanks are perpendicular to the axis of the thread. This is used for transmitting motion or power. E.g. Fly presses, Screw jack, vice handles, cross-slide and compound slide, etc.

Trapezoidal threads: These threads have a profile that is neither square nor V thread form and have a form of a trapezoid. Different forms of trapezoidal threads are ACME thread, Buttress thread, Saw-tooth thread, Worm thread

• ACME thread: It has an included angle of 29°. These are used in lathe lead screws.
• Buttress Thread: In this thread, one flank is perpendicular to the axis of the thread and another flank is at 45°. These threads are used in the power press, carpentry vices, gun breeches, ratchets, etc.
• Knuckle thread: The shape of the knuckle thread is not trapezoid, but it has a rounded shape. It has limited applications. It is used for valve spindles, railway carriage coupling, hose connections, etc.

Whitworth threads: The principal features of the British Standard Whitworth (BSW) thread form are that the angle between the thread flanks is 55 degrees and the thread has radii at both the roots and the crests of the thread.

Among all threads, only Buttress threads do not have a symmetric section.

Concept:

Power Screw: It is a mechanical device used for converting rotary motion into linear transmitting power. It is used in screw jacks, clamps, and vices. 

The screw-in screw-jacks are considered as an inclined plane as shown in the figure below

Then the effort required to raise the load is given by 

P = W tan (α + ϕ)

P = Effort applied at the circumference of the screw to lift the load

W = Load to be lifted

μ = Coefficient of friction, between the screw and nut = tan φ, where φ is the friction angle

α = Helix angle

and the effort required to lower the load is given by 

P = W tan (ϕ - α)

Concept:

Acme threads

• Acme threads can be used to take a load in both directions.
• They are strong, smooth have less wear, and easy to manufacture compared to square thread.

Square thread

• In this thread, the flanks are perpendicular to the axis of the thread.
• This is used for transmitting motion or power. E.g. Fly presses, Screw jack, vice handles, cross-slide and compound slide, etc.
• The efficiency of a square thread is more than that of trapezoidal threads.
• It can transmit load and power in both directions but it is difficult to manufacture.   
• There is no radial or side thrust on the nut.
• The wear on the thread is also a serious problem.

Trapezoidal threads 

• These threads have a profile that is neither square nor V thread form and have a form of a trapezoid.
• Different forms of trapezoidal threads are ACME thread, Buttress thread, Saw-tooth thread, Worm thread

​

• ACME thread: It has an included angle of 29°. These are used in lathe lead screws. 
• Buttress Thread: In this thread, one flank is perpendicular to the axis of the thread and the other flank is at 45°. These threads are used in the power press, carpentry vices, gun breeches, ratchets, etc. It can transmit load and power in one direction only.
• Knuckle thread: The shape of the knuckle thread is not trapezoid, but it has a rounded shape. It has limited applications. It is used for valve spindles, railway carriage coupling, hose connections etc.

Concept:

Screw thread is a continuous helical groove of specified cross-section produced on the external or internal surface. A screw thread formed on a cylinder is known as straight or parallel screw thread, while screw thread formed on a cone or frustum is known as the tapered screw thread.

Crest: Crest of the thread is the top most surface joining the two sides.

Root: Root of the thread is the bottom of the groove between the two flanks.

Pitch: Pitch of a thread is the distance measured parallel to the axis from a point on a thread to the corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis.

Major diameter: It is an imaginary largest diameter of the thread which would touch the crests of an internal or external thread. It is also called crest diameter.

Minor diameter: It is an imaginary smallest diameter of the thread which would touch the roots of an external thread.

Pitch diameter: It is a theoretical diameter between the major and minor diameter of screw threads.

Explanation:

Square threads:

In this thread, the flanks are perpendicular to the axis of the thread. This is used for transmitting motion or power. E.g. fly-presses, screw jack, vice handles, cross-slide and compound slide etc.

Properties:

• The efficiency of a square thread is more than that of trapezoidal threads as its profile angle is zero. 
• It can transmit load and power in both directions.
• There is no radial or side thrust on the nut. It is difficult to manufacture.
• More transmission efficiency due to less friction.
• Load-carrying capacity is low due to less thickness at core diameter than trapezoidal threads.
• The wear on the thread is also a serious problem.

Trapezoidal threads: 

• These threads have a profile that is neither square nor V thread form and have a form of a trapezoid.
• Different forms of trapezoidal threads are ACME thread, Buttress thread, Saw-tooth thread, Worm thread.

Explanation:

Power screw: It is a drive used in machinery to convert rotary motion into linear motion for transmission of power.

The efficiency of a power screw is given by:

\(\eta = \frac{{{\rm{work\;output}}}}{{{\rm{work\;input}}}}\)

\(\eta = \frac{{Fl}}{{P\pi {d_m}}}\)

where, F = force, l = length to which screw travels in the vertical direction 

\(\eta = \frac{{\tan \alpha }}{{\tan \left( {\alpha + \phi } \right)}}\)     

where ⍺ = helix angle or lead angle

Explanation:

Stud:

• A threaded rod, also known as a stud, is a relatively long rod that is threaded on both ends; the thread may extend along the complete length of the rod.
• They are designed to be used in tension.

Additional Information

Rivets: 

• A rivet is a short cylindrical bar with a head integral to it.
• It is a permanent fastening.
• Widely used for joining light metals.
• The cold riveting process is used for the structural joint while hot riveting is used to make leakproof joints.​

Allen bolt:

• A bolt with a hexagonal socket in its head is designed to be used with an Allen wrench.

Explanation:

For a self - locking screw or machine, the coefficient of friction is equal to or greater than the tangent of the helix angle.

In Power screw, the torque M_t required to lower the load is given by

\({M_t} = \frac{{W{d_m}}}{2}\tan \left( {ϕ - α } \right)\)

where, W = load on the screw, dm = mean diameter of the screw, ϕ = friction angle and α = helix angle

Case 1.  when ϕ ≥ α 

tan (ϕ - α ) ≥ 0, thus M_t ≥ 0

i.e. a positive torque is required to lower the load.

Thus the load will not turn the screw and will not descend down on its own unless an effort is applied.

Such cases of screws are knowns as 'Self-locking screws or machines'. And the screw will hold the load without any application of the brake.

Also, as ϕ > α 

tan ϕ > tan α 

\(μ > \frac{l}{{\pi {d_m}}}\)    [∵ μ = tan ϕ]

Case 2. when ϕ < α 

tan (ϕ - α ) < 0, thus M_t < 0

i.e. negative torque is required to lower the load.

The load will turn down the screw and will descend down on its own unless a restoring torque is required to hold the load.

Such cases of screws are known as 'overhauling of the screw' or back drawing of the screw.