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Need some tips in designing a deep ratio helical gear pair

  1. Sep 19, 2013 #1
    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.
     
  2. jcsd
  3. Sep 20, 2013 #2

    Baluncore

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    Science Advisor

    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.
    Code (Text):
    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
     
     
  4. Sep 22, 2013 #3
    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.
    5QCuS0q.jpg
    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!
     
  5. Sep 22, 2013 #4

    Baluncore

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    Science Advisor

    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.
     
  6. Sep 23, 2013 #5
    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. :)
     
  7. Sep 23, 2013 #6

    Baluncore

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    Science Advisor

    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.
     
  8. Sep 23, 2013 #7
    Yeah, I will do that. I will try to find if a ratio more than 6 is used anywhere else as well. :)
     
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