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Statics > Pulleys, Cables, and Friction > Flashcards

Pulleys, Cables, and Friction Flashcards

1
Q

What is a pulley used for?

A

To change the direction of an applied tensile force.

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2
Q

What is a block an tackle?

A

A series of pulleys working together.

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3
Q

What does a block and tackle provide?

A

Pulley advantage.

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4
Q

What is a fixed pulley?

A

A pulley attached by a bracket to a fixed location.

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5
Q

What is a free pulley?

A

If the pulley is attached to a load, or free to move.

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6
Q

What is assumed for pulleys?

A

Friction is disregarded and all ropes are parallel

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7
Q

What is the pulley advantage equal to?

A

The pulley advantage is equal to the number of pulleys coming to and coming from the load carrying pulley.

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8
Q

What is an ideal cable?

A

A cable that is assumed to be completely flexible, massless, and incapable of elongation; it acts as an axial tension member between two points of concentrated loading.

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9
Q

What methods can be used to determine the tensions in cables carrying concentrated loads?

A

The method of joints and sections.

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10
Q

What are the reactions in the cables?

A

All cables will be found to be in tension, and with vertical loads only, the horizontal tension component will be the same in all cable segments.

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11
Q

Are the tensions in every cable segment equal?

A

No, unlike the case of a rope passing over a series of pulleys. The total tension in the cable will be different in every cable segment.

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12
Q

What is friction?

A

Friction is a force that always resist motion or impending motion.

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13
Q

In what direction does friction act?

A

It always acts parallel to the contacting surfaces.

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14
Q

What is static friction?

A

A frictional force applied to a stationary body.

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15
Q

What is dynamic friction?

A

A frictional force applied to a body in motion.

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16
Q

Which one is greater static or dynamic friction?

A

Static friction

17
Q

What is the frictional force dependent on?

A

The normal force and the coefficient of friction between the body and the surface.

18
Q

What is the relationship between magnitudes of static and kinetic friction?

A

The kinetic friction is 75 percent of the static friction.

19
Q

What are the three specific cases of friction?

A

No slip occurring, point of impending slip and slip occurring.

20
Q

What are the magnitudes of the three cases of friction?

A

No slip being less than the frictional force, point of impending slip being equal to the frictional force at static friction and slip occurring being equal to the force at kinetic friction.

21
Q

When a belt is inclined at an angle the normal force is?

A

The product of the normal force and cos of the angle.

22
Q

What does friction between a belt, rope, or band wrapped around a pulley or sheave responsible for?

A

For the transfer of torque.

23
Q

What is the tension distribution when the belt is stationary?

A

The tight side of the belt, will have greater tension than the slack side of the belt.

24
Q

What does the basic relationship between the belt tension and the coefficient of friction neglect?

A

Centrifugal effects.

25
Q

What is the relationship between the tight side and the slack side of a belt?

A

The tension in the tight side is equal to the tension in the slack side plus the effects of the coefficient of friction and the angle of warp expressed in radians.

26
Q

What is the net transmitted torque between the belt?

A

The product of the difference in belt tensions and the radius.

27
Q

What is the power transmitted in a belt running at a tangential velocity?

A

The product of the difference in belt tensions and the tangential velocity.

28
Q

What does a power screw do?

A

It changes rotary motion into transverse motion.

29
Q

How is a square screw thread designed?

A

It is designed using a mean radius, pitch and pitch angle.

30
Q

What is the pitch?

A

The distance between corresponding points on a thread.

31
Q

What is the lead?

A

The distance the screw advances each revolution.

32
Q

How can the coefficient of friction be found for square screws?

A

The coefficient of friction between the threads can be designated directly or by way of a thread friction angle.

33
Q

What is the torque dependent on?

A

The mean thread radius, the torque on the screw, the tensile or compressive force in the screw, the pitch and friction angle.

34
Q

When are the angles added when looking for the square screw torque?

A

For tightening operations.

35
Q

When are the angles subtracted when looking for the square screw torque?

A

For loosening operations.

36
Q

When is the screw self locking?

A

When the tangent of the pitch angle is les than the coefficient of friction between the threads.

37
Q

What is the torque used for?

A

Overcome thread friction and to raise the load

38
Q

What is the mechanical efficiency of the screw equal to?

A

The ratio of the torque without friction to the torque with friction.

Statics (4 decks)

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