Thread: Torque Impulse

Originally Posted by Dougal

I have the inertia of my flywheel, but they are close enough to flat discs to not be affected much. Some of the industrial flywheels feature a large heavy outer ring but these weren't fitted to the truck or landrover motors.

Fair enough - but if, as stated, the Landrover flywheel is larger diameter than the truck one, it will have a larger moment of inertia than the truck flywheel if they are same weight. If the weights are different the moments of inertia need to be calculated and compared, not simply the weights, even if they are simple discs. And then the moment of inertia of the pressure plate has to be added as well, although these probably differ insignificantly (but don't forget the larger diameter driven plate means a larger diameter pressure plate, which is also probably heavier).

John

Originally Posted by rijidij

I didn't actually measure them, but they are different diameters because I remember the Landy flywheel didn't fit in the truck flywheel housing.
I remember at the time thinking it was strange that the Landy flywheel was bigger, yet it has a smaller diameter clutch and pressure plate.

Murray

Thanks Murray.

So it is possible then, that Land Rover either made the flywheel larger to enable the starter motor position to be raised (to clear chassis rail), or they wanted higher flywheel inertia and that is why the starter is higher.

Originally Posted by JDNSW

Fair enough - but if, as stated, the Landrover flywheel is larger diameter than the truck one, it will have a larger moment of inertia than the truck flywheel if they are same weight. If the weights are different the moments of inertia need to be calculated and compared, not simply the weights, even if they are simple discs. And then the moment of inertia of the pressure plate has to be added as well, although these probably differ insignificantly (but don't forget the larger diameter driven plate means a larger diameter pressure plate, which is also probably heavier).

John

Exactly.

Originally Posted by isuzurover

Murray - can you measure the dimensions and weight of your truck flywheel (assuming you still have it???). I can do the same with my landie version.

I will measure and weigh it when I get a chance to move all the other parts that are on top of it

Originally Posted by Bush65

Thanks Murray.

So it is possible then, that Land Rover either made the flywheel larger to enable the starter motor position to be raised (to clear chassis rail), or they wanted higher flywheel inertia and that is why the starter is higher.

The bellhousing bolt PCD's are quite close.
438mm for the LT95 and 425mm for the Isuzu truck.

Here is a drawing using photos and dimensions which I think murray sent me. it's not a real LT95 bellhousing, I just modified the outer ring to suit the lt95 pattern and moved the starter location so it would clear the bellhousing bolts. The obvious omissions are the lt95 housing includes the rear seal housing and has a 3 bolt starter, not the two bolt shown.

http://users.actrix.co.nz/dougal.ell...%20Rev%200.PDF

Originally Posted by garryseries3

So is it the torsional vibrations that destruct gearboxs etc. in Landys or the torque impulse? ...

Probably both.

Torsional vibrations can lead to problems in a lot of rotating machinery. I have seen rotor bars fail in large (over 500kW) electric motors driving ventilation fans due to torsional vibrations from the fans.

Originally Posted by garryseries3

... Perhaps someone could explain torsional vibrations as I though they where much the same. Also does the dampener lead to a quieting motor noise levels?
Kindest regards
Garry

When fuel and air ignites in a cylinder, it applies a force through the piston and conrod to rotate the crankshaft.

This force not only rotates the crankshaft, but twists it between the particular bigend journal and the flywheel.

The force is only applied for a short time and when it diminishes, the twist that was induced, springs back. This continual twist and springback is torsional vibration.

Now the crankshaft will have a natural frequency. If the torsional vibration is far enough away from the natural frequency, the vibration will die away (dampening), until the next cylinder ignites.

If the natural frequency is near the frequency of the torsional vibration (changes with engine revs), the vibration will not die away and will build with further ignitions so the vibration increase dramatically. This effect is resonance (like the string of a musical instrument vibrates at its natural frequency (which varies with tension).

The harmonic balancer is tuned to dampen torsional vibrations near the natural frequency of the crankshaft. The rubber element is designed to have a particular natural frequency. If it has deteriated, its natural frequency will likely change, affecting its ability to dampen the vibration near the natural frequency.

Originally Posted by Bush65

Probably both.

Torsional vibrations can lead to problems in a lot of rotating machinery. I have seen rotor bars fail in large (over 500kW) electric motors driving ventilation fans due to torsional vibrations from the fans.


When fuel and air ignites in a cylinder, it applies a force through the piston and conrod to rotate the crankshaft.

This force not only rotates the crankshaft, but twists it between the particular bigend journal and the flywheel.

The force is only applied for a short time and when it diminishes, the twist that was induced, springs back. This continual twist and springback is torsional vibration.

Now the crankshaft will have a natural frequency. If the torsional vibration is far enough away from the natural frequency, the vibration will die away (dampening), until the next cylinder ignites.

If the natural frequency is near the frequency of the torsional vibration (changes with engine revs), the vibration will not die away and will build with further ignitions so the vibration increase dramatically. This effect is resonance (like the string of a musical instrument vibrates at its natural frequency (which varies with tension).

The harmonic balancer is tuned to dampen torsional vibrations near the natural frequency of the crankshaft. The rubber element is designed to have a particular natural frequency. If it has deteriated, its natural frequency will likely change, affecting its ability to dampen the vibration near the natural frequency.

Thanks John
you explanation gives me a clearer understanding of what this topic is about. So I take it if you have an appropriate harmonic balancer on the isuzu it will dampen the torsional vibrations at low revs (idle). Could I assume some of the noise(Harmonics) effects of the motor at idle would be reduced with the fitting of a harmonic balancer and if one increased the size of the harmonic balancer it may not be of use unless it was tuned to the particular natural frequency, do you think this is a fair assumption?
Kindest regards
Garry

Originally Posted by garryseries3

Thanks John
you explanation gives me a clearer understanding of what this topic is about. So I take it if you have an appropriate harmonic balancer on the isuzu it will dampen the torsional vibrations at low revs (idle). Could I assume some of the noise(Harmonics) effects of the motor at idle would be reduced with the fitting of a harmonic balancer and if one increased the size of the harmonic balancer it may not be of use unless it was tuned to the particular natural frequency, do you think this is a fair assumption?
Kindest regards
Garry

I have no idea of the engine revs that correspond with the natural frequencies of the crank. There will be several natural frequencies but some will be above the peak revs that the engine is capable of. Some may even be below idle speed. I just have no knowledge of the natural frequency that the harmonic balancer is tuned for.

The natural frequency depends upon the stiffness of the member (crankshaft in this case) and the mass. As stiffness increases so does natural frequency (frequency reduces with mass). e.g. tightening a string on a musical instrument makes it stiffer (higher force needed to achieve same deflection) and increases the frequency.

The 1st, 2nd and 3rd natural frequencies are usually the critical ones to be avoided. Generally the stiffness/mass of the member is changed to avoid these.

The torsional stiffness (torque required to create a particular angle of twist) of a crankshaft is difficult for people like us to change.

Changing the size of the harmonic balancer will de-tune it and may even make it worse.

Has anyone determined for sure that the Isuzu engine that Land Rover fitted doesn't have a harmonic balancer? I know that my truck engine has one.

There are places that make harmonic balancers using silicon fluid as the elastic member. These have the advantage of a much wider range of dampening compared to rubber. But unfortunately they don't have any for Isuzu diesels, and wont make custom versions because of the large investment in design required to determine the natural frequencies.

Noise/sound is a vibration transferred through the air to our ears.