Q regarding laser tachometers

[Lloyd]

Hi Gang,

I have a Q about laser tacs. I just bought an Exteck non-contact laser tac. I understand how they work, but my confusion comes from how to actual measure true RPM. If I put the reflector on the timing mark of the front pulley, then won't the RPM be each time the mark comes up?

As apposed to when #1 hits top dead of the compression stroke. Do I factor in and divide by number of cylinders, and is there a factor between 2 stroke and 4 stroke?

Help.

Lloyd

[mobile_bob]

i f you place the reflective tape on a crankshaft driven component, like the flywheel, vibration damper, or front pulley
the laser tach will measure crankshaft rpm, no matter 2 cycle or 4 cycle and no matter how many cylinders.

bob g

[mobile_bob]

and no matter where on the component you put the reflective tape, so basically yes it will read each time the tape flies by
which will tell you how many rpm the component is turning,

so as long as the component is bolted to the crank, it must therefore always turn the same rpm as the crank, so...

you are golden

bob g

[Lloyd]

Maybe that's why I'm so confused....I thought RPM was based on how many #1 compression/firing strokes, so if you have an exhaust stroke and a firing stroke then #1 comes up twice, as well as it seems that 2 inches out from the center of the crank is spinning faster then 15 inches out from the center?

I take your word.

Thanks,

Lloyd

and no matter where on the component you put the reflective tape, so basically yes it will read each time the tape flies by
which will tell you how many rpm the component is turning,

so as long as the component is bolted to the crank, it must therefore always turn the same rpm as the crank, so...

you are golden

bob g

[sailawayrb]

Lloyd, I think maybe you mixing up linear and angular velocity.

Linear velocity (as the name implies) deals with the rate of motion in a straight line (i.e., change in distance per time) with common units such as feet per second or miles per hour.

Angular velocity (as the name implies deals with the rate of motion through an angle (i.e., change in rotation per time) with common units such as radians per second, degrees per second, or Rotations per Minute (RPM).  Recall that the circumference of a circle is 2 Pi radians or 360 degrees.  So, one RPM is equivalent to 2 Pi radians per 60 seconds (about 0.105 radians per second) or 360 degrees per 60 seconds (6 degrees per second).

However, linear velocity and angular velocity are indeed related to each other as you described.  The linear velocity at the surface of a rotating shaft (in units of feet per second) is equal to the shaft's angular velocity (in units of radians per second)  times the distance between the center of the shaft and the surface of the shaft (in units of feet), which is normally known as the radius of the shaft.  So YES, for a given and constant RPM, as the radius increases, so does the associated linear velocity.  At the center of the rotating shaft, both the linear and angular velocities are zero.

As Bob G indicated, the tachometer can only measure the angular velocity of the shaft, i.e., the RPM...given that you placed the reflective tape on the surface of a rotating shaft!  However, knowing the shaft radius and the RPM, one could determine the associated linear velocity at the shaft surface (and really at any distance from the center of the shaft).

For a thought experiment...now let's say you placed the reflective tape on the surface of a linear conveyor belt.  In this case the tachometer could give you the linear velocity of this belt directly (assuming you calibrated it for this purpose).  The tachometer can only measure the time at which the reflective tape moves past it.  It is really the calibration data embedded in the tachometer that determines what it actually reports out.

Bob B.

[Lloyd]

Thank You Bob-G & B,

I'm, really feeling my pie face, in relation to the rule of Pi.

DAMIT janet...that surely is my downfall, I always have to know why, even in the face of the rule of law.

It seems, I learned how to learn, but not how to use what i learned to solve a problem, until someone tells me why I learned what I learned.

hehe the dif..... between a trained monkey, and  an enganier.

Thanks,

Lloyd

Lloyd, I think maybe you mixing up linear and angular velocity.

Linear velocity (as the name implies) deals with the rate of motion in a straight line (i.e., change in distance per time) with common units such as feet per second or miles per hour.

Angular velocity (as the name implies deals with the rate of motion through an angle (i.e., change in rotation per time) with common units such as radians per second, degrees per second, or Rotations per Minute (RPM).  Recall that the circumference of a circle is 2 Pi radians or 360 degrees.  So, one RPM is equivalent to 2 Pi radians per 60 seconds (about 0.105 radians per second) or 360 degrees per 60 seconds (6 degrees per second).

However, linear velocity and angular velocity are indeed related to each other as you described.  The linear velocity at the surface of a rotating shaft (in units of feet per second) is equal to the shaft's angular velocity (in units of radians per second)  times the distance between the center of the shaft and the surface of the shaft (in units of feet), which is normally known as the radius of the shaft.  So YES, for a given and constant RPM, as the radius increases, so does the associated linear velocity.  At the center of the rotating shaft, both the linear and angular velocities are zero.

As Bob G indicated, the tachometer can only measure the angular velocity of the shaft, i.e., the RPM...given that you placed the reflective tape on the surface of a rotating shaft!  However, knowing the shaft radius and the RPM, one could determine the associated linear velocity at the shaft surface (and really at any distance from the center of the shaft).

For a thought experiment...now let's say you placed the reflective tape on the surface of a linear conveyor belt.  In this case the tachometer could give you the linear velocity of this belt directly (assuming you calibrated it for this purpose).  The tachometer can only measure the time at which the reflective tape moves past it.  It is really the calibration data embedded in the tachometer that determines what it actually reports out.

Bob B.

[Thob]

You're probably remembering old electronic tachs for gas engines that counted the pulses on the primary side of the ignition coil.  They typically had switches on them for the number of cylinders and 2/4 cycle setup so they could do the math.  With the cheap ones you had to do the math yourself.