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Reciprocating weight.

  1. Jun 14, 2009 #1
    im trying to find the answer to this question.. "in addition to reducing friction, what other advantage does a roller-type cam follower have in an engine?" i cant decide if this response is correct. "lower reciprocating weight." i know it reduces friction, but does the friction of materials in an engine get calculated into reciprocating weight? i can see how friction is resistance, and if its resistance to the engine reciprocating, than it might as well be like weight. but just because it slows the movement doesnt really mean that makes its calculated reciprocating weight any less right? Unless being a roller-type cam follower is actually lighter than a non-roller type. But I don’t know the specs of the two, and its not given in the question.

    really need some help here.

    thank you.
     
  2. jcsd
  3. Jun 14, 2009 #2
    A roller follower does not a priori reduce reciprocating weight; it actually will slightly increase reciprocating weight, although this is not a significant concern.

    If for example you take a V-type engine with the cam shaft set deep in the valley of the V, so that long push rods are required to operate the rocker arms, etc., then the use of a roller follower is simply a change to the end that is touching the cam and requires more structure on that end than a simple solid lifter.

    If you talk about an overhead cam, you still have the option of a lever type follower or a roller follower, and the lever follower will involve lower mass although the roller follower will last much, much longer.
     
  4. Jun 14, 2009 #3
    i am talking an over head cam. and thats right along my thoughts. theres more mass at the end of the follower because of the roller, so it would make sense that it would not reduce weight. any one have any input? any other theories to back it up or to say he's wrong?
     
  5. Jun 14, 2009 #4
    Did you want to take a vote on this, or did you want information?
     
  6. Jun 14, 2009 #5

    Q_Goest

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    I'd agree with Dr.D's responce. There's nothing intrinsic to the design of a roller follower that results in it weighing less. In fact, it would generally weigh more because of the added complexity.

    On your question: "in addition to reducing friction, what other advantage does a roller-type cam follower have in an engine?" Have you ever looked at a roller follower and compared it to one that just slides? The friction from sliding does more than just generate heat.
     
  7. Jun 14, 2009 #6
    A roller follower, because it does reduce friction, would reduce wear on the cam lobes. This would be important as long as lubricants are insufficient to limit wear on the cam lobes. Furthermore, the heavier reciprocating mass (not weight) would either a) lower the RPM at which the cam follower follows the cam, or b) require slightly stronger valve springs.
     
  8. Jun 14, 2009 #7
    i appreciate your inputs. it would seem the general consensus is that it does not reduce reciprocating weight. thank you all for your responses.
     
  9. Jun 15, 2009 #8

    Ranger Mike

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    In my opinion, having raced both in a 4 cylinder engine..Roller is the only way..You will have zero added reciprocating weight and may even lose some depending on your engine ( 4 cylinder application only, V8 engine is another matter) but parasitic drag may go up. I do not have any empirical data on the torque required to rotate a roller engine vs. flat tappet engine WITH PROPER VALVE SPRINGS INSTALLED. More on this later.

    There are two basic categories among camshafts; flat tappet and roller. Flat tappet cams are the ones most people are familiar with. Most of your V-8 engines in the muscle car era came standard with a flat tappet cam. The tappet, more commonly referred to as the lifter, is, for the most part, flat on the bottom. Oil film is the only thing between the lifter and camshaft lobe preventing them from welding each other together. Eventually, the cam lobes would wear down to a circular shape rather than the teardrop shape they started as. This, in turn, would not open the valves far enough to completely fill the cylinders with a fresh charge of air and fuel or allow the used charge to fully exit through the exhaust. A major loss in power and efficiency was the result. Fortunately, the roller cam almost completely eliminates this wear factor. Roller camshaft lifters are equipped with an actual roller that rides on the cam lobe. This obviously results in a much longer lobe life due to the reduced friction.
    A roller lifter will reduce friction and cam lobe failures.

    In addition to the reliability factor, roller cams also have a great performance advantage over the flat tappet. The faster you can get the intake valve to maximum lift, the longer it can stay there before the piston comes rushing back up the cylinder to compress the air fuel mixture. If the goal is to load the cylinder with as much air and fuel as possible, the longer you are at maximum lift the better. Flat tappets require a smoother transition from the base circle of the cam to max lift, leaving less time for the lifter to dwell there. Roller lifters, on the other hand, can survive a much more abrupt transition resulting in extra time at max lift. If you compare a roller cam lobe to a flat tappet lobe, you can see the roller cam has a larger radius at its peak, holding the lifter at max lift for a longer duration.

    The roller camshaft has steeper opening and closing ramps which allows for more time at max lift.

    With all that said, I think you would agree that a roller cam is the better choice from a reliability and performance standpoint. Now let’s look at some of the things required with the use of a roller cam. To keep lifters on a flat tappet cam wearing evenly, they are designed to rotate in the lifter bore with each revolution of the cam lobe. Not only is this not required on a roller lifter, it is definitely not desirable since rollers only work in one direction. There are two ways to eliminate the rotation of a roller lifter in its bore. On newer blocks that came standard with a roller cam, the top of the lifter bores are machined flat. The upper body of the lifters used with this type of block is also machined flat on two sides. This allows for the use of a special retainer, often referred to as a dog bone, which prevents lifter rotation.

    Now there is a penalty for going the Roller route. Due to the much more aggressive valve opening ramp. stiffer valve springs are required. You can't just slide in the biggest available springs and be done with it? That'll work with a roller camshaft if there is enough room , but for use with a flat tappet camshaft of any sort (hydraulic or solid), too much spring can be worse than too little. Typically, a spring with anything more than 335 pounds of pressure on the nose (open pressure) will rapidly increase the wear on a flat tappet camshaft (along with wear in other areas such as cast-iron guides). Depending on the application design, the cam profile and the valve train geometry, the practical limit for open pressure on a flat tappet cam is approximately 375 pounds. Any more and you'll probably be faced with a pile of broken camshaft pieces.

    Simply stated, heavier springs require more horsepower to move the valve train. But there's more to the "too much is just right" scenario. If (and it's a big IF) the engine can physically live with a large amount of open spring pressure, you're still behind the 8 Ball. Why? Large amounts of valve spring pressure can eat horsepower. Increasing the open pressure by 50% also increases the amount of friction inside the engine. Simply stated, heavier springs require more horsepower to move the valve train. In the end, oil temperature increases and power levels can drop by five, six or more horsepower.

    In effect, selecting valve springs is much like walking a tightrope. Too little seat pressure creates an open invitation to valve bounce and float. Too much open pressure can physically break the camshaft. In order to arrive at a happy (and workable) medium, the correct spring for the intended RPM range should be selected.
     
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