Camshafts

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Camshafts
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Camshaft

• The camshaft rotates ½ times the crankshaft or
  
• once per four-cycle stroke.
• The camshaft may operate the
• Valve train
• Mechanical fuel pump
• Oil pump
• Distributor

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Camshaft

• Major function - operate the valve train.
• The lobes on the cam open the valves against the
  pressure of the valve springs.
• Bearing journal can be internally or externally
  lubricated (oiled).

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When installing externally oiled cam bearings it
is essential that the holes in the bearings
lineup with the oil passages in the block
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Camshaft

• Pushrod engines have the cam located in the
  block.

• Cam is supported by the block and the cam
  bearings.

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• Cam may or may not be held in place by a thrust
  plate.

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Overhead Camshafts

• Overhead camshafts are either belt or chain
  driven and are located in the cylinder heads.

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Overhead Camshafts

• May be housed within a bore in the cylinder head
  or
• May be supported within a cradle held in place
  with caps.
• May or may not utilize a thrust plate.
• Either design may or may not use separate cam
  bearings

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Overhead Camshafts

• Will use one of the following
• Cam followers
• Rocker arms
• May have a one piece lifter rocker design
• A bucket design

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Bucket Design
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Camshaft Followers
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Rocker Arms
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Design

• A cam casting will include
• Lobes
• Bearing journals
• Drive flanges
• A cam casting may include
• Drive gear(s)
• Fuel pump eccentric

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Classification

• Camshafts are of one of four types
• Hydraulic flat-tappet
• Hydraulic roller
• Solid flat-tappet
• Solid roller
• This designation is actually determined by the
  lifter design.

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Hydraulic flat-tappet

• The lifter is spring and oil loaded to allow
  for compensation.
• Traditional O.E. style (1950s mid 90s)
• Used with flat or convex-faced lifters
• Generally cast iron or hardened steel
• Requires a break-in period to establish a wear
  pattern

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Hydraulic flat-tappet

• Cast-iron cams are finished with a phosphate
  coating.
• Steel cams (SAE 4160 or 4180) are hardened by
• Induction hardening heated cherry-red in an
  electric field then oil cooled.
• Liquid nitriding hardens to .001 to .0015
• Gas nitriding hardens to .004 to .006 thickness

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Flat tappet Lifters
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Hydraulic flat-tappet

• Most cams are coated at the factory with
  manganese phosphate . This gives the cam a dull
  black appearance. This coating is to absorb and
  hold oil during the break-in period.

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Hydraulic flat-tappet

• Most late model designs use a convex bottom
  (.002) to encourage lifter rotation.
• This rotation helps reduce lifter and (or) bore
  wear.
• The Cam lobe will also be slightly tapered
  (.0007 - .002).
• This provides for a wider contact pattern.

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Hydraulic flat-tappet

• Camshaft break-in
• The lobes of the cam and the bottom of the
  lifters must be coated with a molydisulfide
  lubricant often called cam lube.
• This insures that the cam is properly lubricated
  during break-in.

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Hydraulic flat-tappet

• Camshaft break-in
• Typical procedure
• Maintain 1,500 RPM for 10 - 20 minutes
• Drain the engine oil a immediately afterwards
• Check the recommended procedure and lube for your
  particular cam!

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Hydraulic Roller

• The lifter is spring and oil loaded to allow
  for compensation.
• The contact between the cam and lifters are
  separated by a steel roller.
• This roller reduces friction.
• Lifters cannot be allowed to rotate within the
  lifter bore.

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Hydraulic Roller

• The camshaft is generally made of non-hardened
  steel.
• The lobes must be finished by the manufacturer
  prior to assembly
• there is no break-in period.

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Hydraulic Lifters (tappets)

• Hollow cylinders fitted with a plunger, check
  valve, spring and push-rod seat.

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Hydraulic Lifters (tappets)

• Engine oil pressure forces oil into the lifter
  through the oil inlet holes.
• A check valve and ball hold most of the oil
  inside the lifter hydro-locking the plunger
  inside the cylinder.

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Hydraulic Lifters

• The oil passed through the check valve exits
  through the hole in the push rod seat.
• The oil then passes through the pushrod to
  lubricate the rocker arms.

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Hydraulic Lifter Preload

• Also called valve lash.
• The distance between the pushrod seat and
  snap-ring when the lifter is resting on its base
  circle.
• Typical values range from .020 to .045.
• Check manufacturers specifications.

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Hydraulic Lifter Preload

• Adjusted by
• Adjustable rocker arms
• Often referenced by turns past zero lash
• Non-adjustable rocker arms
• Longer or shorter pushrods
• Shim or grind rocker stands

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Hydraulic Lifter Preload

• Necessary if
• Cylinder head has been decked
• Cam has been changed
• Altered head gaskets
• Camshaft is worn
• An engine rebuild

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Hydraulic Lifter Valve-floatNOT GOOD

• The lifter fills with oil faster than it can
  purge it. This raises the lift of the camshaft.
• Usually caused by excessive RPM.
• May damage valves, pushrods, pistons etc.

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Solid Flat-tappet and Roller

• No internal hydraulic absorption.
• Allows for a more consistent valve lift,
  especially at high RPM.
• Noisy when cold, more frequent and precise
  valve-lash adjustments required.

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Solid Flat-tappet and Roller

• Oil is diverted through the pushrods via a
  pushrod seat.

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Solid Flat-tappet and Roller

• No lifter preload valve lash only.
• Lash values may be given hot or cold
• Typical values range from .002 - .005.

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Composite Approaches

• Composite camshafts of medium- and high-alloyed
  powered metal lobes mounted on a hollow tube are
  popular as they have the capacity of withstanding
  high contact stresses. Composite camshafts can be
  50 lighter than cast iron or steel shafts can
  have lobe material tailored to the application
  and they can have lobes molded to near-net shape,
  which means that the amount of grinding stock,
  and consequently grinding time, are both reduced.
  

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Cam Specifications

• Lift
• Duration
• Valve overlap
• Lobe center (separation angle or lobe spread)

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Lobe Lift

• The amount the cam lobe lifts the lifter
• Expressed in decimal inches
• As lift increases the forces on the entire valve
  train also increase.

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Lobe Lift

• Asymmetrical design the amount of lift between
  the intake and exhaust lobes is different.
• Symmetrical design - the amount of lift between
  the intake and exhaust lobes is the same.

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Duration

• The number of degrees of crankshaft rotation for
  which the valve is lifted off of the seat.
• If the amount of degrees that the intake and
  exhaust valve are open differ it is of an
  asymmetrical design.

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Duration

• Usually expressed as one of two values
• Duration (at zero lash)
• Duration at .050 lift preferred method
• Compensates for tappet styles and clearances

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Duration

• More duration rougher idle and better high RPM
  performance
• Less duration smoother idle and better low RPM
  performance

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Valve Overlap

• The number of degrees of crankshaft rotation that
  both valves are off of their seat (between the
  exhaust and intake strokes).
• Lower overlap a smoother idle and better low
  RPM operation
• Higher overlap better high RPM operation

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Valve Overlap

• Having the exhaust valve still open when the
  intake starts to open uses the exhaust "pull" out
  the exhaust port to help start the intake charge
  entering the chamber -- before the piston has
  started down and has generated it's own vacuum.

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Valve Overlap
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Lobe Separation Angle

• The difference, in degrees, between the center of
  the intake lobe and the center of the exhaust
  valve.
• The smaller the angle the greater the overlap
• The larger the angle the less the overlap

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Lobe Separation Angle
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Camshaft Degreeing

• Advanced cam timing
• The camshaft is slightly ahead of the crankshaft
• More low speed torque
• less high RPM power
• Retarded timing
• The camshaft is slightly behind the crankshaft
• More high RPM power
• Reduced low RPM torque

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Adjustable Camshaft Gear
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