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Mechanisms and Tolerances

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Mechanisms and Tolerances. By: Jose Camero. Jeremiah Hernandez. Samantha Guerra. Carlos Anzaldua ... A mechanism is a system of parts that are connected ... – PowerPoint PPT presentation

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Title: Mechanisms and Tolerances


1
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

3
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

4
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.

5
Mechanism Make-up
  • The two mechanisms are made up of several
    different connections.
  • Cylinder Connection
  • Pin Connection
  • Planar Connection

6
Step by Step Connection
  • Step 1
  • Crankshaft
  • Uses a pin connection
  • Pin connections require axis alignment and a
    translation alignment.

7
Step by Step Connection
  • Step 2
  • Piston Sub-Assembly
  • (3) individual parts
  • Piston Head
  • Pin
  • Connecting Rod

8
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.

9
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.

10
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.

11
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.

12
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.

13
Part 2 Tolerances
14
Tolerances
  • Tolerance is the total amount a dimension may
    vary.
  • You calculate the tolerance by subtracting the
    maximum and minimum limits.

15
(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

16
Lawn Mower Engine Applications
  • Tolerances are used to manufacture
  • Piston head
  • Piston rings
  • Cylinder bore
  • Precision drill holes

17
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

18
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20
Piston Ring Clearance
  • Minimum bore diameter
  • 120.00 mm
  • Maximum piston head diameter
  • 119.880 mm
  • Clearance for piston rings
  • 0.12 mm

21
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23
Tolerance Stack-Up
  • Can affect all parts of the engine.
  • Occurs when tolerances to the same point are
    taken from different directions.

24
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
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