190201h Flashcards
On engines with UNIT fuel injectors the cam also actuates
the injector operating mechanism
the cam is driven by the crank and changes motion from what to what?
rotary motion into reciprocating
Cam to followers
Cam nose is a mounting surface for the drive gear or sprocket. What maintains alignment of the gear to cam
a keyway is machined into the nose to retain the key
What are the load bearing surfaces of the cam
Journals. they support the cam in the block
Lobes are eccentrics that convert the rotary motion of the cam to…
reciprocating motion of the cam follower
depending on the engine, the cam gear train may also run the
fuel injection pump, fuel transfer pump, oil pump , tach
what is a helical gear machined into a cam most likely to run?
oil pump or tach
the nose on a cam lobe is…
the point of maximum lift
the centerline is the
point between opening and closing ramps
what part of the lobe does a lifter ride up and down on?
the opening and closing ramps. there is one on all lobes
what is the base circle
the base diameter of the lobe and no valve is open on it
what math formula on what parts gives you calculated lift?
subtract base circle from the lobe height
What is the definition of “Duration”
the number of degrees between where the valve opens to the point where it closes
What are the three ways camshafts are manufactured
Casting, machine or forged from alloyed steel and assembled from components
Why are cast camshafts heat treated
To improve durability
When new most camshafts have a black coating on the lobes. What is it?
Pressure resistant antiwear agents molybdenum disulfide or manganese phosphate
Where would you use a steel camshaft
Where extreme loads are placed on the camshaft such as with unit fuel injectors
Why would you use a modular camshaft
In some large displacement diesel engines or to permit ease of manufacturing and service
What are the two general camshaft locations
In block or overhead
Camshaft motion is transmitted how with the in block design
Through the cam follower and pushrod to the rocker arm
What are the advantage of the overhead camshaft
Fewer moving parts, less friction, more accurate valve timing
What type of bushings are generally used to support the camshaft and control and play
Friction type bearings
How are bushings normally retained in the cylinder block
Bushings are retained by a press fit in the cylinder block cam bearing bores
Many diesels with overhead camshaft use what type of bearings
Shell type bearings. Which permits the camshaft to be removed vertically from the cylinder head
True or false. The camshaft is one of the last components to receive oil
True
What are three different ways to limit end play of the camshaft
Taper of the camshaft lobes. Cam shaft driven by helical gears. A thrust plate or washer is the most common method of controlling and play on diesel engine camshafts
What two ways are camshaft lobes lubricated by oil
Sprayed out of the camshaft bearing and splashed off the valve operating mechanism
What’s the purpose of a cam follower
Turn the rotary motion of the camshaft into reciprocating motion by following the profile of the cam lobe
What are the three basic types of cam followers
Solid, roller and hydraulic
What are two other names for cam followers
Lifters or tappets
What are two types of flat faced solid lifters
Straight body and mushroom
What is the advantage of a mushroom type solid lifter over a straight body lifter
Mushroom type have a greater surface area in contact with the cam lobe and will therefore withstand higher contact pressure
What are three ways a solid lifter is kept from wearing quickly
The lifter has a concave, the lobe has a taper, the lifter centerline is also offset in relation to the cam lobe creating rotation
A solid cam follower has no mechanism to compensate for heat expansion therefore what must be in place
There must be some clearance or valve lash to allow for expansion as temperature increases
Roller lifters can either be a solid or hydraulic design. What are three characteristics
The cam lobe for a lifter is ground flat and has no taper. The roller is usually centered on the cam lobe for Full Contact. It cannot turn during operation and will have a positioning mechanism to prevent it from turning
Roller type cam followers reduce what three things
They reduce friction, wear, and sides thrust on the lifter body
What are three advantages of roller type cam followers
Reduced frictional loss, can handle much higher operating loads, allow a quicker rate of valve opening and closing then solid cam followers
Overhead cams have what built into the rocker arm
A cam follower roller
What are two things a hinged roller eliminates
The need for lifter bores in the cylinder block and side thrust created by conventional roller followers
What engine design uses the hinged roller
Primarily used on engines with mechanically actuated fuel injectors. It opens both valves and injectors
What are the two advantages of hydraulic lifters over solid lifters
Hydraulic operated lifters compensate for heat expansion therefore no valve lash is necessary and hydraulic lifters operate with no valve clearance and all valvetrain components are in continual contact during engine operation resulting in less valvetrain noise
What are two ways timing gears are installed on the camshaft and two ways that they are indexed
They are bolted or pressed on and indexed by a keyway or Dowe
If the physical distance between crank and cam is too great what’s in between
An idler gear that does not affect speed relationship but does change direction of rotation
Timing chains are used on both in block and overhead cam applications what are two types of chains
Silent type chain for light and medium duty and roller type chain for heavy duty applications
On four stroke cams intake and exhaust lobes are offset. what is this referred to as
Lobe separation angle
What is one of the most critical steps in reassembling an engine
Timing the camshaft
Incorrect camshaft timing will result in what four things
Altered valve opening and closing points, poor engine performance, damage to engine components due to physical contact, alter fuel injection timing on engine camshaft actuated injectors
What other engine accessories may the camshaft gear drive
Fuel injection pump,oil pump, water pump,accessory drives and balance shafts
What may be used on the camshaft drive gear instead of multiple timing marks
Some engines use offset keys to change the camshaft timing
