1.In the unified designation of 4-40 UNF thread, what do the numbers 4 and 40 re
ID: 1843717 • Letter: 1
Question
1.In the unified designation of 4-40 UNF thread, what do the numbers 4 and 40 represent respectively?
A. The nominal major diameter and the thread per inch
B. The nominal major diameter and the pitch
C. The size and the thread per inch
D. The size and the pitch
E. None of the above
2.What of the following statements is true about ACME threads?
ACME threads are used when rapid movement is required
ACME threads are used to transmit large forces
ACME threads are easy to machine
ACME have low efficiencies
All of the above
3.What of the following statements is true about ball screws?
Ball screws require less power to drive the load
Ball screws present less friction
Balls, in ball screw mechanisms, eliminate the “slip-stick” motion caused by friction
Ball screws usually need a break to hold the load in place
All of the above
4.Which of the following statements is true about the lead angle of a screw mechanism?
Small lead angles are desired for lifting devices
The torque required to push the load along the screw increases with the lead angle
The lead angle is the angle of inclination of the thread(s)
Small lead angle combined with friction make the screw mechanism self-locking
All of the above
5.What of the following is true about the efficiency of a screw mechanism?
The efficiency is the ratio of torque required to raise the load without friction to that required to raise the same load with friction
The efficiency is function of the pitch diameter
The efficiency is function of the lead
The efficiency is the percent
6.Which of the followings statements is true about a differential screw mechanism?
Differential screws are a type of auger screws
Differential screws are a type of Archimedes screw
Differential screws motion is less fine than ball screws
Differential screws are designed to provide very fine motion
None of the above
7.Which of the following statements is NOT true about auger screw mechanism?
The volume through auger screw is inversely proportional to the screw angular displacement
The mating nut is the material transported
The volume flow rate through an auger flow is proportional the screw angular speed
Material transported is trapped between auger blades
The volumetric transport rate is function of the clearance between auger blades
8.Which of the following is NOT true about screw mechanisms?
Special surface treatments and coatings can reduce the coefficient of friction at the thread-nut interface
The force and torque of a screw mechanism is strongly function of the coefficient of friction between the thread and the nut
For a self-locking crew mechanism, the lead angle is greater than the friction angle
The volumetric acceleration through an auger screw is proportional to the screw angular acceleration
Ball screws are virtually never self-locking
Book:Machines & Mechanisms- 4th edition
Explanation / Answer
1) (E). There is no thread designated as 4-40 UNF. There exist 4-40 UNC, where 4 denotes nominal major diameter of 0.1120 inches and 40 denotes thread per inch. There exist a thread designated as 4-48 UNF.
2) (E). Acme threads can be machined using single point cutting tool, so they are easy to machine. Also there exist large friction forces during operation using acme threads, so efficiency is low.
3) (E) Due to the presence of rolling friction, the power required to drive the load is less and also rolling friction is less than sliding friction
6) (D) Differential screw is used for very fine motion becouse of the use of two screw threads of different leads. Thus allowing very small moment and adjustment of the distance.
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