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Millwrong > Balancing > Flashcards

Balancing Flashcards

1
Q

With rotating machinery, ____ is the unequal distribution of weight of a rotating body about its centreline.

A

imbalance

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2
Q

What type(s) of forces does machine imbalance create on the rotating mass?

A

Reciprocating forces

This is vibration.

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3
Q

How do corrosion and abrasion cause machine imbalance?

A

Corrosion and abrasion result in wear.

This changes the distribution of weight in the machine.

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4
Q

Fan blades may expand when running, and due to initial stresses and variations in material thickness, the distortion caused by this expansion is considered asymmetrical.

  1. What effect does that have on the balance of the fan?
  2. Why is that?
A
  1. It unbalances it.
  2. The asymmetrical distortion causes some fan blades to move further from the center axis than others.
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5
Q

In rotating machinery, ________ occurs when the rotating centreline of the rotor is not the same as the shaft axis.

A

eccentricity

This appears as a 1x rpm frequency.

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6
Q

True or false: No ball bearing manufacturer guarantees the bore of the inner race or the groove on its circumference for perfect concentricity.

A

True

This can result in imbalance due to eccentricity.

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7
Q

What is the most common cause for a bent shaft?

A

Internal stresses from the manufacturing process.

These are stresses locked up in the shaft at the time of manufacture from grinding, heat treating, or other manufacturing operations.

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8
Q

True or false: Accumulation of tolerances during assembly can cause the pulley, gear or coupling to be off-centre with the rotor centreline, resulting in imbalance.

A

True

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9
Q

Why is it important to use the key that is provided with motors?

A

To maintain machine balance.

Using a different key could result in imbalance.

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10
Q

What is non-homogeneity of materials?

A

Where the properties of a material (such as density) are not uniform throughout its volume.

With vibration, things such as blowholes or slag inclusions can cause the weight of the part to become imbalanced.

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11
Q

True or false: Asymmetrical weight distribution is the basic cause of imbalance.

A

True

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12
Q

Rotating equipment has the natrual tendency to accumulate material build up and deposits inside sheaves and rotors. What affect does this have on the objects balance?

A

It will become imbalanced.

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13
Q

What is shaft flexing?

A

Deflections for different shaft or rotor positions in machinery having asymmetrical shafts or bodies

This variation may cause vibration of a different frequency at high speeds.

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14
Q

Why is the placement of bearing pillow block housings important?

A

It should be as close to the asymmetrical shaft variances as possible

This helps to dampen vibrations.

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15
Q

True or False: Shaft flexing only affects low-speed machinery.

A

False

Shaft flexing can cause vibration at high speeds.

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16
Q

Why is it impossible to balance any machine that is used in liquid collecting?

A

The liquid (weight) moves

This results in the weight of the rotating mass always moving when at balancing speeds.

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17
Q

What are the three types of rotating machinery imbalance?

A
  1. Static
  2. Couple
  3. Dynamic
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18
Q

This type of rotor imbalance is caused when a shaft’s weight is evenly distributed along its length, but unevenly from side to side.

A

Static imbalance

The weighted centreline and the shaft axis are parallel, but shifted along the length.

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19
Q

With this type of rotor imbalance, the principle inertia axis (weighted centreline) and the shaft axis are parallel, but shifted along the length.

A

Static imbalance

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20
Q

With this type of shaft imbalance, the heavy side settles at the bottom of rotation.

A

Static imbalance

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21
Q

What is single plane balancing?

A

Where the required balance correction is at a single point on the rotor.

This is seen in rotors that are statically imbalanced.

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22
Q

This type of rotor imbalance is caused by equal weight at opposite ends of the shaft, 180º apart.

A

Couple imbalance

23
Q

What effect does couple imbalance have on the principle inertia axis (weighted centreline) of the rotor?

A

The weighted centreline tilts or shifts relative to the rotors centreline

Unlike static imbalance, couple imbalance does not shift the center of mass off the axis of rotation. The center of mass remains aligned with the axis, but the mass is distributed unevenly.

24
Q

This type of rotor imbalance is caused by extra weight unevenly distributed along the length of the shaft, the width of the shaft, or at different angles about the radius of the shaft.

A

Dynamic imbalance

25
Q

With dynamic imbalance, the extra weight on the rotor can be distributed in what three ways?

A
  1. Along the length of the shaft
  2. Across the width of the shaft
  3. At different angles about the radius of the shaft.
26
Q

____ imbalance is caused by the presence of two weights in separate planes, diametrically opposite each other.

A

Couple imbalance

27
Q

What effect does couple imbalance have on the rotor as it turns?

A

It causes the rotor to rock or twist

This twisting or rocking happens on an axis perpendicular to the axis of rotation.

28
Q

In balancing, when components or assemblies of significant length require two weights at opposing ends to balance the rotor, this is known as:

A

Dual plane (two plane) balancing

This is done with couple and dynamic imbalances.

29
Q

What is pure dynamic imbalance?

A

The same as couple imbalance.

This is caused by the presence of two weights in separate planes, diametrically opposite each other.

30
Q

True or false: The amount of force caused by the lack of balance of a rotor is proportional to the rpm.

A

False

Force is proportional to the rpm squared.

31
Q

When using a stroboscope triggered by a vibration transducer for balancing, the distance that the heavy spot moves past the pickup before the flash occurs is called the:

A

flash angle

This is the angle of delay (distance) between the heavy point and the reference.

32
Q

In balancing operations, what is the phase lag?

A

The same as the flash angle.

The spot where the line remains is called the phase angle.

33
Q

True or false: A stobe’s light primary purpose in balancing is to check phase angle, which requires the end of the shaft to be visible.

A

True

34
Q

If the vibration pickup on a machine is moved from horizontal to vertical (H to V) and the get 90º phase shift 90% of the time, this shift is caused by:

A

imbalance

If you get 180º phase shift, the problem is not imbalance; it is more likely shaft misalignment.

35
Q

True or false: A body is in balance when the centre of gravity of its mass coincides with the axis in which it rotates.

A

True

36
Q

How is imbalance on a rotating body measured?

A

ounce-inches (oz-in) or gram-centimetres (g-cm)

One oz-in = 72 g-cm

37
Q

In balancing, one oz-in = ____ g-cm.

A

1 oz-in = 72 g-cm

38
Q

A common approach for selecting a trail weight when balancing a rotor is to use a weight that produces an imbalance force at the support bearing equal to ____% of the rotor weight supported by the bearing.

A

10%

This can be done by using the F = 1.77 x R x W x (rpm ÷ 1000)² formula and knowing the approximate weight of the rotor.

39
Q

What are three pieces of information that are needed to calculate the trail weight of a rotor?

A
  1. Weight of the rotor
  2. Rotor rpm
  3. Radius of the trial weight
40
Q

What formula can be used to calculate trail weights when balancing a rotor?

A

F = 1.77 x R x W x (RPM ÷ 1000)²

F = force in lb
R = radius in inches
W = weight in oz
RPM = rpm of the rotor

41
Q

When balancing a rotor, if it is not possible to place the correction weight at the same radius as the trial weight, what two thing must happen?

A
  1. The weight angle must stay the same
  2. The product of the trial weight angle and the correction weight angle must be equal to the produce of the adjusted weight and new radius*

*Point two means that the oz-in ratio must be equal ⟶ 3.5oz x 10” = 35oz-in, is the same as 5oz x 7” = 35oz-in.

42
Q

When balancing, a ________ is a line whose length corresponds to the amount of vibration (imbalance) present and whose direction corresponds to the phase angle.

A

vector

43
Q

True or false: When a balancing question lists the correction angle, that is where the weight would be placed.

A

True

44
Q

True or false: When a balancing question lists the unbalanced angle, that is where the weight would be placed.

A

False

It would be 180º opposite from the unbalanced angle.

45
Q

What effect does rotor speed have on balancing?

A

The higher the speeds, the more precise the balancing needs to be.

46
Q

True or false: Couple imbalance is created by equal heavy spots at each end of the rotor, but on opposite sides of the centreline.

A

True

47
Q

True or false: With couple imbalance, there is a 180º phase difference between rotor ends.

A

True

48
Q

True or false: When balancing, the reference mark does not change when the trial weight location is changed.

A

False

The reference mark changes if the heavy weight changes.

49
Q

If the set screw on a hub causes the runout, what multiple of RPM would show on a vibration signature?

A

1x rpm

50
Q

True or false: Correction weights should be placed at the angle determined and at the same radius from the centre as the trial weight.

A

True

51
Q

If the rotor on a motor has two separate locations of imbalance in separate planes, what type of imbalance is present?

A

Dynamic imbalance

52
Q

What multiple of RPM does a static imbalance cause on a vibration frequency signature?

A

1x rpm

53
Q

When determining if a vibration is caused by an imbalance, moving the transducer from the horizontal plane to the vertical plane should cause the phase to shift by ________°, 90% of the time.

A

90º

54
Q

True or false: Machine parts that have been produced from cast iron can result in a non-homogenous structure that may result in an imbalance.

A

True

Millwrong (27 decks)

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