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Questions on engine RPM

Discussion in 'Mechanic and Repair' started by Juan_Deere, Oct 16, 2007.

  1. Juan_Deere

    Juan_Deere LawnSite Senior Member
    Posts: 301

    If I measure engine RPM, using a laser type tachometer at the screen covering the flywheel/cooling fan, am I getting a true RPM reading? Or do I need to measure it at the output shaft? Or somewhere else altogether?

    I have been having a striping problem(unmowed stripes) with my mower. I got the majority of it fixed by leveling the deck, and a company rep sent me some blades to help troubleshoot. I put the blades on, I think they are Gators, and I think they are so heavy and create so much drag that they caused the striping to come back way worse than the OEM blades did, by(it seems to me) reducing the engine RPM severely. I checked the RPM and I am getting a reading at about 3550. The manual says the engine operates at 3600. When I engage the OEM blades, not even mowing yet, just engaging the PTO in the garage, the RPMs drop to 3520 or so.

    My main question being. Is that 50 RPM, at full throttle, enough to show up or manifest itself as a mowing quality problem? Or is that considered within a tolerable range? I know I can just bump it up and check, but I can't really verify for sure, because the grass is pretty much going dormant for the year. So I can't get a real world, everyday cutting observation until spring time.

    Any experience in the matter is much appreciated.

    The engine is a Kohler Courage, 23HP, with about 125 hours on it. Purchased new in March of this year.
  2. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Although I have never used one of these type tachs I would think it wouldn't be any difference between the fan or shaft.

    You could bump the rpm up to 3600 with the PTO engaged and still be OK but try not to run wide open without the PTO on.
  3. Juan_Deere

    Juan_Deere LawnSite Senior Member
    Posts: 301

    Yeah, the tach is actually pretty cool. Although there are improvements that could be made in the design. All you do is, place a piece of reflective tape on a point where you want to measure and point the laser at it. We actually use a similar device when checking the track of rotor blades on helicopters.

    Gravely striping problem.jpg
  4. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    OOOK, Now you have my interest since my boss owns a two place helicopter. How exactly do you do something like this with a laser ? Is it done with the blades engaged and at what RPM ? Is this something that should be performed at the major annual inspection ?

    His is downed right now for annual and was just wondering if this is something they do.

    Sorry for all the questions !

  5. Juan_Deere

    Juan_Deere LawnSite Senior Member
    Posts: 301

    On a helicopter, the track is usually checked if a pilot reports an unusual vibration or beat, or if a rotor blade is changed. I don't believe it is done a regular check, such as an annual. But it may be on different helicopters than the ones I worked on.

    It is performed with the rotor spinning at full speed. On the ground, in a hover, and at predetermined airspeeds. It's actually a bit more complicated than just pointing a laser at the blades though. There are also vibrations that have to be measured and brought within spec. There are basically accelerometers throughout the aircraft that tell you what the vibes are, and an optical eye that shows you the blade split as the plane is flying, and a computer that tells you what adjustments to make, so that everything will come into synch.
  6. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Interesting info there, They are having to replace one blade due to the leading edge having corrosion under the cap. I will ask him about this tracking test just to find out if they did it.

    I've done quite a bit of work on this helicopter;


    Had someone told me I'd be working on one 20 yrs. ago I probably would have called them a liar.....

  7. Juan_Deere

    Juan_Deere LawnSite Senior Member
    Posts: 301

    LOL. That helicopter is a lawn mower compared to the helicopter I worked on. I have moved over to the data analysis side of the house, instead of turning wrenches but I don't rule out going back to the "dirty" side of the house.

    APP Systems 057.jpg
  8. Restrorob

    Restrorob LawnSite Fanatic
    Posts: 11,023

    Hey....That isn't a helicopter you posted, That's a freakin AIR-SHIP LOL.....

    Boss's son is in the Air Force spinning wrenches on those BIG double rotor jobs (forget the name). He has applied for flight training to fly one but I'm un-sure of his status right now.

    Myself, I keep these two feet planted safely on the ground....Less distance to fall. :laugh:

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