Mechanisms and Tolerances

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Mechanisms and Tolerances

• By
• Jose Camero
• Jeremiah Hernandez
• Samantha Guerra
• Carlos Anzaldua

2
Outline

• Part 1
• Mechanisms
• Associated Connections

• Part 2
• Tolerances
• Lawn Mower Engine Applications

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Definition

• A mechanism is a system of parts that are
  connected together in order to perform a certain
  task.
• Lawn Mower Engine Mechanisms (2)
• Gear Mechanism
• Cam Follower Mechanism

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Engine Mechanisms Examples

• Gear Connection The principle connection that
  initializes the engine movement.
• Cam-follower Connection uses the movement
  provided by the two gears to transmit radial
  movement into vertical movement.

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Mechanism Make-up

• The two mechanisms are made up of several
  different connections.
• Cylinder Connection
• Pin Connection
• Planar Connection

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Step by Step Connection

• Step 1
• Crankshaft
• Uses a pin connection
• Pin connections require axis alignment and a
  translation alignment.

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Step by Step Connection

• Step 2
• Piston Sub-Assembly
• (3) individual parts
• Piston Head
• Pin
• Connecting Rod

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Piston Sub Assembly

• The piston head uses one cylinder connection for
  vertical alignment of the piston head and the
  cylinder bore.
• The pin uses a pin connection.
• This requires a horizontal axis alignment between
  the pin axis and the top axis on the connecting
  rod.
• Translation along the center datum of the pin and
  the center length-wise datum of the connecting
  rod is also required.

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Piston Sub Assembly

• The connecting rod also uses a pin connection.
• The bottom axis of the connecting rod is
  horizontally aligned with the center axis of the
  crankshaft.
• Translation was aligned using the center
  length-wise datum of the rod with a datum created
  on the crank that was aligned with the center of
  the cylinder bore.

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Cam Pin Connection

• Step 3
• The camshaft
• The camshaft requires a pin connection
• The horizontal axis is aligned with the
  horizontal axis from a hole in the back of the
  engine case.
• The translation is between the inside surface of
  the hole in the engine and the outer flat surface
  of the shaft.

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The Lifters

• Step 4
• The lifters
• These require a only one cylinder connection.
• The center axis of the lifter is aligned with the
  center axis of the designated hole.
• This connection is needed to provide a range of
  vertical movement that the cam will deliver after
  the engine has started to turn.

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The Intake/Exhaust Valves

• Step 5
• The two lifters require a planar connection along
  with a cylinder connection.
• The cylinder connection allows the valve to
  vertical move in its designated hole.
• The planar connection allows the lifter below it
  to provide upward movement transferred from the
  camshaft.

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Part 2 Tolerances
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Tolerances

• Tolerance is the total amount a dimension may
  vary.
• You calculate the tolerance by subtracting the
  maximum and minimum limits.

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(2) Types of tolerances

• Unilateral tolerance varies in only one
  direction
• 22 0.2
• Bilateral tolerance varies in both direction
  from the basic size.
• If the variation is equal in both directions,
  then the variation is preceded by a plus and
  minus symbol.
• 22 0.2

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Lawn Mower Engine Applications

• Tolerances are used to manufacture
• Piston head
• Piston rings
• Cylinder bore
• Precision drill holes

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Piston-to-Wall Clearance

• Free Running Fit large temperature variations
  and high speeds.
• Clearance Fit a space is always left between the
  piston head and the cylinder walls.
• Basic sizes
• Cylinder bore 120 mm
• Piston head diameter 120 mm

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Piston Ring Clearance

• Minimum bore diameter
• 120.00 mm
• Maximum piston head diameter
• 119.880 mm
• Clearance for piston rings
• 0.12 mm

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Tolerance Stack-Up

• Can affect all parts of the engine.
• Occurs when tolerances to the same point are
  taken from different directions.

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Tolerance Stack-Up Effects

• Cylinder will not be square with crankshaft
• Extra friction on the interior cylinder wall
• Irregular cylinder wall wear overtime
• Extra heat will cause a drop off in performance