Power Screws MCQ Quiz in తెలుగు - Objective Question with Answer for Power Screws - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Concept:

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.

The threads given in question i.e. Square, Acme and Buttress all can be used in power transmission.

Explanation:

Pitch:

• The distance from one point on a thread to the same point on the adjacent thread. The reciprocal of the pitch is the number of threads per inch (TPI).

Thread:

• Threads are formed on screws in a helix.
• Helix is the path of a point travelling around an imaginary cylinder such that its axial and circumferential velocities maintain a constant ratio. 
• When a single helix is making a screw, it is called a single start thread. In a single start thread, the lead and pitch are the same. 
• 
• In the case of two start (double start) threads, one thread is wound within the other exactly in the middle. This enables the lead of the helix increased without increasing the pitch. 
• Lead = Pitch x Number of starts 
• 
• A screw-thread may have any number of starts. The general term for such threads other than single start is multi-start.
• Application of multi-start threads can be found in fly presses, pen caps etc. A multi-start thread makes it possible to keep the depth of Thread less, and provides a rapid axial movement of the screws.

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:

Designation of screw thread

The size of the screw thread is designated by the letter ‘M’ followed by the diameter and pitch, the two being separated by the sign ×.

Example:

M 33 × 2

• M → metric thread
• 33 → nominal diameter
• 2 → pitch

Explanation:

ISO Designation of trapezoidal threads:

Tr 40 x 14 (P7): 

• Multiple start trapezoidal threads are designated by letter 'Tr'
• Followed by the nominal diameter and the lead seperated by sign x
  • ​​Nominal diameter = 40 mm
  • Lead = 14 mm
• The letter P followed by the pitch expressed in mm.
  • ​Pitch = depth/2 = 14/2 = 7 mm​

Trapezoidal thread:

• Trapezoidal threads are used for power transmission.
• They are manufactured on a thread milling machine. 
• The axial wear on the surface of trapezoidal threads can be compensated by means of a split-type of nut. It is used in the lead screw of a lathe to compensate wear at periodic intervals by tightening the two halves.

Explanation:

Screws

The screws are used to convert rotary motion into translatory motion. There are various types of screws:

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, Lead screw, presses and clamping devices.

Additional Information

Some other types of screw are:

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 the 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.
• Ex. 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 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:

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

Calculation:

Given:

\(\eta = \frac{{1 - sin\phi }}{{1 + sin\phi }} = \frac{{1 - \sin 30^\circ }}{{1 + \sin 30^\circ }} = \frac{{1 - \frac{1}{2}}}{{1 + \frac{1}{2}}} = \frac{1}{3} = 33.3\% \)

Explanation:

Square and trapezoidal threads are the 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 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.
• 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.

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 the 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:

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

Important Points

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.