Need some tips in designing a deep ratio helical gear pair

[k.udhay]

Hi,

For a final drive of a light commercial truck, I have to design a final drive helical gear pair. The expected ratio is 8.2. The centre distance is 134 mm. The input max. torque is 120 Nm. I have sized few combinations.

I have a feeling that this ratio is too deep. Pl. share me your opinions and views on this. I mainly need few check-points while designing such a deep ratio helical gear pair. Thanks.

 

[Baluncore]

What do you mean by the ratio being too "deep".
The final drive usually refers to the differential.
Do you need a 90° angle between the gear shafts?

The pinion is usually hypoid, running on a simple flat faced tooth crown wheel.

Here is a table of ratios that can be used. Avoid common ÷ divisors.

Hunting Tooth.    Differential Gear Ratios without Common Factors.

      Pinion    5      6      7      8      9     10     11     12     13
Crown

  20           ÷ 5    ÷ 2   2.857   ÷ 4   2.222   ÷ 10  1.818   ÷ 4   1.538
  21          4.200   ÷ 3    ÷ 7   2.625   ÷ 3   2.100  1.909   ÷ 3   1.615
  22          4.400   ÷ 2   3.143   ÷ 2   2.444   ÷ 2    ÷ 11   ÷ 2   1.692
  23          4.600  3.833  3.286  2.875  2.556  2.300  2.091  1.917  1.769
  24          4.800   ÷ 6   3.429   ÷ 8    ÷ 3    ÷ 2   2.182   ÷ 12  1.846
  25           ÷ 5   4.167  3.571  3.125  2.778   ÷ 5   2.273  2.083  1.923
  26          5.200   ÷ 2   3.714   ÷ 2   2.889   ÷ 2   2.364   ÷ 2    ÷ 13
  27          5.400   ÷ 3   3.857  3.375   ÷ 9   2.700  2.455   ÷ 3   2.077
  28          5.600   ÷ 2    ÷ 7    ÷ 4   3.111   ÷ 2   2.545   ÷ 4   2.154
  29          5.800  4.833  4.143  3.625  3.222  2.900  2.636  2.417  2.231

  30           ÷ 5    ÷ 6   4.286   ÷ 2    ÷ 3    ÷ 10  2.727   ÷ 6   2.308
  31          6.200  5.167  4.429  3.875  3.444  3.100  2.818  2.583  2.385
  32          6.400   ÷ 2   4.571   ÷ 8   3.556   ÷ 2   2.909   ÷ 4   2.462
  33          6.600   ÷ 3   4.714  4.125   ÷ 3   3.300   ÷ 11   ÷ 3   2.538
  34          6.800   ÷ 2   4.857   ÷ 2   3.778   ÷ 2   3.091   ÷ 2   2.615
  35           ÷ 5   5.833   ÷ 7   4.375  3.889   ÷ 5   3.182  2.917  2.692
  36          7.200   ÷ 6   5.143   ÷ 4    ÷ 9    ÷ 2   3.273   ÷ 12  2.769
  37          7.400  6.167  5.286  4.625  4.111  3.700  3.364  3.083  2.846
  38          7.600   ÷ 2   5.429   ÷ 2   4.222   ÷ 2   3.455   ÷ 2   2.923
  39          7.800   ÷ 3   5.571  4.875   ÷ 3   3.900  3.545   ÷ 3    ÷ 13

  40           ÷ 5    ÷ 2   5.714   ÷ 8   4.444   ÷ 10  3.636   ÷ 4   3.077
  41          [B]8.200[/B]  6.833  5.857  5.125  4.556  4.100  3.727  3.417  3.154
  42          8.400   ÷ 6    ÷ 7    ÷ 2    ÷ 3    ÷ 2   3.818   ÷ 6   3.231
  43          8.600  7.167  6.143  5.375  4.778  4.300  3.909  3.583  3.308
  44          8.800   ÷ 2   6.286   ÷ 4   4.889   ÷ 2    ÷ 11   ÷ 4   3.385
  45           ÷ 5    ÷ 3   6.429  5.625   ÷ 9    ÷ 5   4.091   ÷ 3   3.462
  46          9.200   ÷ 2   6.571   ÷ 2   5.111   ÷ 2   4.182   ÷ 2   3.538
  47          9.400  7.833  6.714  5.875  5.222  4.700  4.273  3.917  3.615
  48          9.600   ÷ 6   6.857   ÷ 8    ÷ 3    ÷ 2   4.364   ÷ 12  3.692
  49          9.800  [B]8.167[/B]   ÷ 7   6.125  5.444  4.900  4.455  4.083  3.769

  50           ÷ 5    ÷ 2   7.143   ÷ 2   5.556   ÷ 10  4.545   ÷ 2   3.846
  51         10.200   ÷ 3   7.286  6.375   ÷ 3   5.100  4.636   ÷ 3   3.923
  52         10.400   ÷ 2   7.429   ÷ 4   5.778   ÷ 2   4.727   ÷ 4    ÷ 13
  53         10.600  8.833  7.571  6.625  5.889  5.300  4.818  4.417  4.077
  54         10.800   ÷ 6   7.714   ÷ 2    ÷ 9    ÷ 2   4.909   ÷ 6   4.154
  55           ÷ 5   9.167  7.857  6.875  6.111   ÷ 5    ÷ 11  4.583  4.231
  56         11.200   ÷ 2    ÷ 7    ÷ 8   6.222   ÷ 2   5.091   ÷ 4   4.308
  57         11.400   ÷ 3   [B]8.143[/B]  7.125   ÷ 3   5.700  5.182   ÷ 3   4.385
  58         11.600   ÷ 2   [B]8.286[/B]   ÷ 2   6.444   ÷ 2   5.273   ÷ 2   4.462
  59         11.800  9.833  8.429  7.375  6.556  5.900  5.364  4.917  4.538

  60           ÷ 5    ÷ 6   8.571   ÷ 4    ÷ 3    ÷ 10  5.455   ÷ 12  4.615
  61         12.200 10.167  8.714  7.625  6.778  6.100  5.545  5.083  4.692
  62         12.400   ÷ 2   8.857   ÷ 2   6.889   ÷ 2   5.636   ÷ 2   4.769
  63         12.600   ÷ 3    ÷ 7   7.875   ÷ 9   6.300  5.727   ÷ 3   4.846
  64         12.800   ÷ 2   9.143   ÷ 8   7.111   ÷ 2   5.818   ÷ 4   4.923
  65           ÷ 5  10.833  9.286  [B]8.125[/B]  7.222   ÷ 5   5.909  5.417   ÷ 13
  66         13.200   ÷ 6   9.429   ÷ 2    ÷ 3    ÷ 2    ÷ 11   ÷ 6   5.077
  67         13.400 11.167  9.571  [B]8.375[/B]  7.444  6.700  6.091  5.583  5.154
  68         13.600   ÷ 2   9.714   ÷ 4   7.556   ÷ 2   6.182   ÷ 4   5.231
  69         13.800   ÷ 3   9.857  8.625   ÷ 3   6.900  6.273   ÷ 3   5.308

 

[k.udhay]

Thank you Baluncore. This is for a final drive inside transaxle with input and output on one plane. Hence it is a helical gear pair.

My fear is if the targeted ratio 8.2 is too high to achieve... If we assume 49 teeth for bull gear and 8 for pinion teeth, do I have to anticipate any specific problems? Thanks again!

 

[Baluncore]

You will have a problem. A basic helical gear is designed using the same fundamental rules as those of a spur gear. The number of teeth on the pinion will be a minimum determined by tooth profile, maybe about 13, which will require the bull gear to have 106 or 107 teeth.

The advantage of the 90° hypoid is the very low number of teeth needed on the pinion while still avoiding tooth interference. I would expect a light truck to have a final drive ratio of about 4.7, maybe slightly lower with a torque converter. You should check your calculation of the 8.2 ratio. You should also check the size and possibility of reducing the road wheel diameter.

If you examine examples of the ratios used by the gear manufacturer in a manual gearbox, you will find the larger gear (on the mainshaft) is driven by the first gear pinion (part of the layshaft/countershaft). The tooth count on the first gear pinion will be very close to what is achievable on your final drive.

 

[k.udhay]

Hi Balluncore,
Sorry if I am beating about the bush... What if I use 16t in pinion and 131t in bull gear? Since I have a good centre distance of 134mm, I can have a transverse module of 1.8mm (approx.). If my calculation shows there is no undercut, what do you feel about this combination.
Thanks for your helps in many of my posts. :)

 

[Baluncore]

I feel the individual teeth will be too small to handle the expected torque. The gear will have to be very wide.
You need to examine a manual gearbox 1'st gear pinion from a vehicle with similar performance.

 

[k.udhay]

Yeah, I will do that. I will try to find if a ratio more than 6 is used anywhere else as well. :)