FMTV High Speed Gears

[scottmandu]

Where do you get this info.? The chunk is marked 3.07 and the housing is marked 6.14 finial drive.

Refer to my previous post and count the teeth. The bevel gears are a 14 and 24 tooth. Doing the math yields a 1.71 ratio.

Only thing I can figure is either ratios for bevel gears is a different calculation or the parts book for the bevel gears is incorrect.

I do recall Meritor axle information that stated the hub reduction could be 1.7:1, 2:1 or 2.14:1.

 

[NDT]

Please study this: http://www.odts.de/southptr/gears/gears.htm

Of importance is the section E discussion of reduction.
Also important is the observation: "The size of wheel 3 does not influence the behavior of the differential. It can therefore be chosen as space (or torque) dictate"

It's taken me all evening to find an adequate explanation of the bevel gear hub reduction, so I hope you will take a minute to study this.

 

[Suprman]

I read it. Over my head though.

 

[Duckworthe]

Guys, now I will never be able to sleep again! Here I was worrying about driving my truck today or tomorrow. Now I have to worry about what my final ratio is!? Sheesh. Of course I'm kidding. But I understand, it is important to others.

 

[Suprman]

I want to know first because I part trucks and sell axles and I want to give buyers correct info. And its good info to know anyways.

 

[ChiefMinion]

For the record, I don't own an FMTV (yet). I follow this forum because I would like one and want to get informed. Also, I'm sure I'll regret commenting on this. At this point for me, it's just an interesting physics problem.

I have read the gearing tutorial, and bookmarked it. Thanks for posting that. It is a good reference.

Respectfully, I suggest that the 1.71 argument is flawed. This is because the spider gear assembly drives the wheel, not the annulus gear. I argue that the arc covered by the spider gear assembly will always be 1/2 the arc covered by the sun gear (drive shaft) regardless of the tooth counts on any of the gears. So, I argue 2:1 reduction.

If by some mechanism, the spider gear assembly were fixed to the front axle and the annulus gear fixed to the wheel, then the 1.71:1 ratio would be correct.

Again, just MHO. There is a definitive test. If someone has a diff cover removed and the axle jacked up, turn the wheel one rotation and observe how many rotations the drive shaft makes. Can't argue with that.

More generally, thank you to all of you who are so free with your expertise. I've learned a lot!

 

[scottmandu]

For the record, I don't own an FMTV (yet). I follow this forum because I would like one and want to get informed. Also, I'm sure I'll regret commenting on this. At this point for me, it's just an interesting physics problem.
I have read the gearing tutorial, and bookmarked it. Thanks for posting that. It is a good reference.
Respectfully, I suggest that the 1.71 argument is flawed. This is because the spider gear assembly drives the wheel, not the annulus gear. I argue that the arc covered by the spider gear assembly will always be 1/2 the arc covered by the sun gear (drive shaft) regardless of the tooth counts on any of the gears. So, I argue 2:1 reduction.
If by some mechanism, the spider gear assembly were fixed to the front axle and the annulus gear fixed to the wheel, then the 1.71:1 ratio would be correct.
Again, just MHO. There is a definitive test. If someone has a diff cover removed and the axle jacked up, turn the wheel one rotation and observe how many rotations the drive shaft makes. Can't argue with that.
More generally, thank you to all of you who are so free with your expertise. I've learned a lot!

The tooth counts do come into play however I am unsure of the calculation. The bevel gear setup is essentially two cones and the diameter of those cones determines the gear ratio. By making the spiders larger, they are covering more distance per revolution, and by making the outer gears smaller there is less distance to cover per revolution.

I've just not had the time to do the math or study the link posted.

 

[Suprman]

Meritor lists the standard ratios for the fmtv axles as 6.14, 7.10 and 7.80. It has to be one of them.

 

[DiverDarrell]

Well here's the deal. I'm going to do the center section swap when the weather gets better. With the front end on jack stands and drive shaft disconnected wheels and drums off. I will pop the cover off one side and put the cutout cover back on. And tape a degreed circle to the drive shaft. On the other side I will tape a degrees circle to the cover. Then set indicators to see how much rotatin happens when I turn the drive shaft flange one rotation. So what we will be able to get is final ratio, and hub ratio. I will do this with both normal and high speed gears in place. With lots of photos.

 

[ramdough]

Well here's the deal. I'm going to do the center section swap when the weather gets better. With the front end on jack stands and drive shaft disconnected wheels and drums off. I will pop the cover off one side and put the cutout cover back on. And tape a degreed circle to the drive shaft. On the other side I will tape a degrees circle to the cover. Then set indicators to see how much rotatin happens when I turn the drive shaft flange one rotation. So what we will be able to get is final ratio, and hub ratio. I will do this with both normal and high speed gears in place. With lots of photos.

You will need to measure the rotation at both hubs simultaneously. With open differentials, one side can either increase or decrease the rotation of the other. If you measure both you can tell if the differential is messing with your results.


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[expeditionnw]

You will need to measure the rotation at both hubs simultaneously. With open differentials, one side can either increase or decrease the rotation of the other. If you measure both you can tell if the differential is messing with your results.


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If one tire is one the ground, it will not move. Therefore no power can be transferred thru that side of the axle. Jacking up one tire will render the results needed.

 

[steve6x6x6]

Meritor lists the standard ratios for the fmtv axles as 6.14, 7.10 and 7.80. It has to be one of them.

3.07 x 2 to1 is =6.14.; 3.55 x 2 to 1=7.10.; 3.90 x 2 to 1= 7.80 finial drive.

 

[ramdough]

If one tire is one the ground, it will not move. Therefore no power can be transferred thru that side of the axle. Jacking up one tire will render the results needed.

According to my memory......the above is Not true...... if one tire is not moving, half of the differential will be fixed and the differential will double the rpm on the spinning output side. At least that is my recollection of what happens.

Steve would probably have a better explanation.

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[ramdough]

According to my memory......the above is Not true...... if one tire is not moving, half of the differential will be fixed and the differential will double the rpm on the spinning output side. At least that is my recollection of what happens.

Steve would probably have a better explanation.

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I had more time to think about it. I am convinced my statement is true. Basically the pinion gear drives the ring. The ring spins the carrier (excuse me if my terms are off in this discussion). The carrier spins the spider gear shaftes. Now if both axles are spinning at the same rate, the axles spin at the same rate as the carrier/ring and the spider gears do not spin on their shafts. If one axle is not moving at all, and the carrier is spinning, the gear on one of the axle shaft is stationary, the carrier is spinning at the speed of the ring along with the spider shafts, then the output spinning axle rpm is doubled because the stationary axle gear back drives the opposite axle gear through the spinning spider gears.

Does that make sense? I might have to word this again once I have thought more about it.


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[spankybear]

Meritor part number B416561

This seems to be export controlled or ITAR. Has anybody purchased these? Also are these the 3.07 ratio?

 

[scottmandu]

This seems to be export controlled or ITAR. Has anybody purchased these? Also are these the 3.07 ratio?

According to Meritor that is the correct part number for the 3.07 ratio ring and pinion. There are none in stock but they are still a valid part number and can be manufactured.

 

[DrillerSurplus]

I had more time to think about it. I am convinced my statement is true. Basically the pinion gear drives the ring. The ring spins the carrier (excuse me if my terms are off in this discussion). The carrier spins the spider gear shaftes. Now if both axles are spinning at the same rate, the axles spin at the same rate as the carrier/ring and the spider gears do not spin on their shafts. If one axle is not moving at all, and the carrier is spinning, the gear on one of the axle shaft is stationary, the carrier is spinning at the speed of the ring along with the spider shafts, then the output spinning axle rpm is doubled because the stationary axle gear back drives the opposite axle gear through the spinning spider gears. Does that make sense? I might have to word this again once I have thought more about it. Sent from my iPhone using Tapatalk

It has been a while since I've done it, but from checking ratios on other vehicles over the years, I think you are correct.
I also recall that rather than trying to make sure both sides were turning at same rate due to differences in brake drag, etc., I found it was easier to be accurate by keeping one side stationary and divide by two.

 

[ramdough]

It has been a while since I've done it, but from checking ratios on other vehicles over the years, I think you are correct.
I also recall that rather than trying to make sure both sides were turning at same rate due to differences in brake drag, etc., I found it was easier to be accurate by keeping one side stationary and divide by two.

Makes a lot of sense.


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[dirtdustand4x4]

Can the 6x6 MTV be regeared with high speed gears? I'm looking at M1088's but I'm not sure I'm interested at 55mph. I might just live with the lighter weight M1078 and get the higher top speed.

 

[dronsen]

No problem to re-gear 6x6, but obviously need to replace all three gear sets