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#1
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Mechanical tach failure?
So, I was flying back home, happily burping all the shellfish I ate at the
Shelton Oysterfest, when my engine started speeding up. Well, it didn't really - but my tach said it did. Without touching the throttle, I went from 2400 rpm up to 3400. Funny, my airspeed didn't change. It looks like my sturdy, simple mechanical tach (bought used at OSH) has gone South on me. I pulled it out and removed the case. Everything inside looks great. It is cable driven through a magnetic coupling just like a car speedo. The needle has a return spring wound in a coil like a clock spring. I assume the calibration is done by setting the needle on its shaft in a position relative to the return spring, so a given rpm matches the needle position on the dial. I thought I would discover a broken return spring, but it looks okay. Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial. If I allow the needle to bypass the stop pin and add an extra full turn of tension on the return spring, the tach shows the drill turning ~400. It looks like the needle has slipped on the shaft releasing about a half-turn of spring tension. Either that or A. the spring has developed a weak spot or B. the other end of the spring has come unsoldered from the frame. I don't think it's "B" because the entire spring would be unwound. Visual examination of the spring doesn't show any defects. Any ideas? Rich "May as well take it apart - it don't work" S. |
#2
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Or the needle slipped on the shaft?
Rich S. wrote: So, I was flying back home, happily burping all the shellfish I ate at the Shelton Oysterfest, when my engine started speeding up. Well, it didn't really - but my tach said it did. Without touching the throttle, I went from 2400 rpm up to 3400. Funny, my airspeed didn't change. It looks like my sturdy, simple mechanical tach (bought used at OSH) has gone South on me. I pulled it out and removed the case. Everything inside looks great. It is cable driven through a magnetic coupling just like a car speedo. The needle has a return spring wound in a coil like a clock spring. I assume the calibration is done by setting the needle on its shaft in a position relative to the return spring, so a given rpm matches the needle position on the dial. I thought I would discover a broken return spring, but it looks okay. Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial. If I allow the needle to bypass the stop pin and add an extra full turn of tension on the return spring, the tach shows the drill turning ~400. It looks like the needle has slipped on the shaft releasing about a half-turn of spring tension. Either that or A. the spring has developed a weak spot or B. the other end of the spring has come unsoldered from the frame. I don't think it's "B" because the entire spring would be unwound. Visual examination of the spring doesn't show any defects. Any ideas? Rich "May as well take it apart - it don't work" S. |
#3
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"Rich S." wrote in message ... So, I was flying back home, happily burping all the shellfish I ate at the Shelton Oysterfest, when my engine started speeding up. Well, it didn't really - but my tach said it did. Without touching the throttle, I went from 2400 rpm up to 3400. Funny, my airspeed didn't change. It looks like my sturdy, simple mechanical tach (bought used at OSH) has gone South on me. I pulled it out and removed the case. Everything inside looks great. It is cable driven through a magnetic coupling just like a car speedo. The needle has a return spring wound in a coil like a clock spring. I assume the calibration is done by setting the needle on its shaft in a position relative to the return spring, so a given rpm matches the needle position on the dial. I thought I would discover a broken return spring, but it looks okay. Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial. If I allow the needle to bypass the stop pin and add an extra full turn of tension on the return spring, the tach shows the drill turning ~400. It looks like the needle has slipped on the shaft releasing about a half-turn of spring tension. Either that or A. the spring has developed a weak spot or B. the other end of the spring has come unsoldered from the frame. I don't think it's "B" because the entire spring would be unwound. Visual examination of the spring doesn't show any defects. Any ideas? Rich "May as well take it apart - it don't work" S. An old gent I know who has been flying longer than anybody offered the observation that the reason that old airplanes don't fly as fast as new ones is that the needle return springs on tachometers and manifold pressure gauges get weak with age causing the instrument to over-indicate. If pilots keep setting power at the same gauge indications the airplane will indeed fly slower as it ages. Re-calibrating the gauges restores the youthful vigor. If only that worked for me. Bill Daniels |
#4
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"Bill Daniels" wrote in message
news:Kjdid.351704$MQ5.71307@attbi_s52... An old gent I know who has been flying longer than anybody offered the observation that the reason that old airplanes don't fly as fast as new ones is that the needle return springs on tachometers and manifold pressure gauges get weak with age causing the instrument to over-indicate. If pilots keep setting power at the same gauge indications the airplane will indeed fly slower as it ages. Re-calibrating the gauges restores the youthful vigor. If only that worked for me. Bill................ You mean that what I got is catching - and my tach come down with it? Oh, Lordy. Actually the tach started acting up all of a sudden, where my body took years to reach its present state of weakness. Rich "Need some vigor-ah" S. |
#5
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You can take it to an automotive speedometer shop and they can clean,
lubricate, and calibrate. Tell them that it is driven off the cam in your stock car so they get it right and don't panic. It is standard old style AC and they uncrimp and re-crimp the bezels all the time. -- Cy Galley - Chair, AirVenture Emergency Aircraft Repair A Service Project of Chapter 75 EAA Safety Programs Editor - TC EAA Sport Pilot "Rich S." wrote in message ... So, I was flying back home, happily burping all the shellfish I ate at the Shelton Oysterfest, when my engine started speeding up. Well, it didn't really - but my tach said it did. Without touching the throttle, I went from 2400 rpm up to 3400. Funny, my airspeed didn't change. It looks like my sturdy, simple mechanical tach (bought used at OSH) has gone South on me. I pulled it out and removed the case. Everything inside looks great. It is cable driven through a magnetic coupling just like a car speedo. The needle has a return spring wound in a coil like a clock spring. I assume the calibration is done by setting the needle on its shaft in a position relative to the return spring, so a given rpm matches the needle position on the dial. I thought I would discover a broken return spring, but it looks okay. Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial. If I allow the needle to bypass the stop pin and add an extra full turn of tension on the return spring, the tach shows the drill turning ~400. It looks like the needle has slipped on the shaft releasing about a half-turn of spring tension. Either that or A. the spring has developed a weak spot or B. the other end of the spring has come unsoldered from the frame. I don't think it's "B" because the entire spring would be unwound. Visual examination of the spring doesn't show any defects. Any ideas? Rich "May as well take it apart - it don't work" S. |
#6
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"Cy Galley" wrote in message
news:eAeid.52917$R05.12713@attbi_s53... You can take it to an automotive speedometer shop and they can clean, lubricate, and calibrate. Tell them that it is driven off the cam in your stock car so they get it right and don't panic. It is standard old style AC and they uncrimp and re-crimp the bezels all the time. Cy........... You the man! Rich S. |
#7
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On Wed, 3 Nov 2004 13:23:05 -0800, "Rich S."
wrote: So, I was flying back home, happily burping all the shellfish I ate at the Shelton Oysterfest, when my engine started speeding up. Well, it didn't really - but my tach said it did. Without touching the throttle, I went from 2400 rpm up to 3400. Funny, my airspeed didn't change. It looks like my sturdy, simple mechanical tach (bought used at OSH) has gone South on me. I pulled it out and removed the case. Everything inside looks great. It is cable driven through a magnetic coupling just like a car speedo. The needle has a return spring wound in a coil like a clock spring. I assume the calibration is done by setting the needle on its shaft in a position relative to the return spring, so a given rpm matches the needle position on the dial. I thought I would discover a broken return spring, but it looks okay. Turning the drive with an 1100 rpm electric drill shows ~2500 on the dial. If I allow the needle to bypass the stop pin and add an extra full turn of tension on the return spring, the tach shows the drill turning ~400. It looks like the needle has slipped on the shaft releasing about a half-turn of spring tension. Either that or A. the spring has developed a weak spot or B. the other end of the spring has come unsoldered from the frame. I don't think it's "B" because the entire spring would be unwound. Visual examination of the spring doesn't show any defects. Any ideas? How about either a change inmagnetic flux or a change in the position of the EC (Edy Current) ring? A weakened magnet would cause the reading to DROP. So something that causes the aluminum disc to get closer to the magnet would cause the RPM to read high, or something that causes the air movement from the spinning magnet to influence the disc - like a bit of oil or grease from the cable migrating into the instrument. Rich "May as well take it apart - it don't work" S. |
#8
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Cy Galley wrote:
: You can take it to an automotive speedometer shop and they can clean, : lubricate, and calibrate. Tell them that it is driven off the cam in your : stock car so they get it right and don't panic. : It is standard old style AC and they uncrimp and re-crimp the bezels all the : time. Isn't the aircraft tach run off the cam, too? Thus indicating 2500 RPM would require the cable at 1250? Whatever you do... don't tell them it's for an airplane... -Cory -- ************************************************** *********************** * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * ************************************************** *********************** |
#9
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"Jerry J. Wass" wrote in message
... DON'T TAKE IT APART !! At least not until you have lubed the drive cable.. when they get dry, they start sticking somewhere, the part in front of the "stick" winds up a little, then releases this energy in a quick snap, this causes the instrument to read high. I know this may sound silly, but it happens every winter with my trucks. TOO LATE !! But I got it back together. Even used all the pieces Actually, you anticipated my next thought. I got to wondering if I was assuming too much by blaming the tach head. I'm going to reassemble the unit in the case and lube the cable. Thanks, Jerry. Rich S. |
#10
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wrote in message
... How about either a change inmagnetic flux or a change in the position of the EC (Edy Current) ring? A weakened magnet would cause the reading to DROP. So something that causes the aluminum disc to get closer to the magnet would cause the RPM to read high, or something that causes the air movement from the spinning magnet to influence the disc - like a bit of oil or grease from the cable migrating into the instrument. Like I said, everything looked fine inside (no dirt or grease). There is a scuff mark on the inside of the disk which could be new or old. If lubing the cable doesn't do the trick, I'll probably send it out to be rebuilt; unless the cost is more than another used unit. Thanks, Rich S. |
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