Worm Gears

Worm Gears
    Worm gears are the perfect choice when the need is for producing large motor speed Gear reductions in a single step. A single reduction range of 5:1 - 100:1 is considered quite a normal thing for Worm Gears. It can climb up to 127:1 or even more, under special circumstances. Worm Gears are quite in operation. Using a single-lead worm along with a 100 tooth wheel, can do wonders in a single simple step.

    Actually by taking a low motor out put of 100 rpm, it can give you 1 rpm with 100 times of the torque (approx). This means that if two of these gear sets is put in series, a whopping 10,000:1 speed reduction, is not a remote possibility any more. This can drive, for example a huge telescope with even a tiny motor. For safety linked systems, a worm should not be applied as a braking device, for example as in hoists.

Types of Worm Gears

For the purposes of industrial application there are three different kinds of Worm Gears.

      Non Throated Worm Gears
      In non-throated worm gears, both the worm and the driven gear is not throated. The image of a non-throated worm Gear is depicted below.

       Single Throated Worm Gears
In the Single-throated worm gears one element that is generally the driven gear is throated. Tooth contact takes place in a single moving point on a worm drive.

        Double Throated Worm Gears
In case of double-throated worm gears, it is the driven gear as well as the worm that gets throated. Higher loads are permissible without undergoing excessive wear.



Manufacturing Process of Worm Gears

Worm Gears can be formed by the process of milling the teeth on a special type of machine. They are then consequently carburized, thread ground and hobbed to size. Manufacturing materials include cast iron or Bronze.

Characteristics of Worm Gears

The Worm Gears shows the following distinct characteristics:

    * Worm gears are cut Helically for maximum mating
    * Perfect for accurate movement of load
    * Occurrence of pure sliding motion
    * Single step conversion of high speed inputs to low speeds and high Torque outputs
    * Large speed reductions

Factors affecting the Worm Gear efficiency

The following factors affect Worm Gear efficiency:
» Lead angle of the worm
» Installation conditions
» Sliding Speed
» Quality of the Surface
» Choice of Lubricant

Improving the Worm Gear Efficiency

These days by using a latest technology called the Double enveloping, the efficiency of the Worm gears have been greatly enhanced. Double-enveloping technology for worm gearing has given numerous distinct advantages over the normal cylindrical worm gearing. This includes great features like an increased torque throughput, more life, better accuracy. Let's analyse each of the features of double enveloping and find out what makes them special.

    * Design of Double Enveloping Worm Gear
      The design made for the double-enveloping worm gearing is very unique in many aspects. It is completely based on the novel tooth form concept. Actually the double enveloping Worm gear Technology has straight-sided forms on both of the gear teeth. This is a better option than involute or other types of curved tooth forms.

    * Backlash
      It has been seen that Double-enveloping worm gearing offers a very low backlash as compared with the traditional cylindrical worm gearing. Now this low backlash invariably translates into more accuracy than those obtained from cylindrical worm gearing.

    * Better Lubrication
      In the double enveloping technology, there is an increase in the number of gear teeth that comes in touch with worm threads. This property has the capacity to lessen the load at any given point on each of the gear tooth, thereby significantly improving lubrication.

    * Better Contact Patterns
       The diagram here depicts the contact pattern for a Gear tooth using superior Cone Drive double enveloping worm gearing. It can be seen that the portion between the third and the fifth Gear teeth lies in mesh with the worm at any given point of time. In the ongoing mesh cycle, the improved geometry of the double-enveloping worm gear forms two lines of contact on every single gear tooth. In normal cases of cylindrical Worm Gears it is just one. The gear tooth is then rotated through its own arc of contact with the worm. This leads to one line of contact being maintained at the center of the tooth. While the other contact line then sweeps from the left hand side of the tooth towards the center. As the tooth's movement ends through the contact arc (see positions 4 and 5), the two distinct lines of contact now converges into a single line at the center of the tooth. This line of contact is more or less perpendicular to the worm thread sliding action.

Advantage of Double Enveloping

It is reported that the benefits derived from the double-enveloping design are truly dramatic. One advantage is that the total load here, gets divided among individual gear teeth. As a result of this, there is considerable enhancement in the load carrying capacity. Secondly, there is improvement in the torque throughput. This allows for a smaller size reducer to even produce the same amount of torque. This leads to size and weight savings.

Ratio     Cylindrical Worm For 4'' center distance     Double enveloping Worm gear                                               For 4'' center distance
10:1         10.71 HP @ 1750 RPM
        3304 lb.In.Output Torque         8.40 HP @ 1750 RPM
                            6190 lb.In.Output Torque
25:1:00     5.08 HP@ 1750 RPM
        3743 lb.In.Output Torque         8.13 HP@ 1750 RPM
        6370 lb.In.Output Torque

Lubrication for Worm Gears

Lubrication for Gear oil is a challenge in itself. In worm gears there is continuous sideway sliding motion. This makes it hard in maintaining a hydrodynamic oil wedge. The result is gears operating under conditions of boundary lubrication. Also, high operating temperatures that sometimes borders on 190°F (88°C) and even higher generally requires oil with a specification of ISO VG of 460 (AGMA Class 7) or even higher. These oils also need good oxidative and thermal stability.

The types of lubricants that are commonly applied for worm gears are the following:

    * Compounded mineral oils
      - Finds extensive use in worm gears
      - The temperature limitation is around 180°F (82°C)
      - Normal use of AGMA Class 7 or Class 8 compounded oils (ISO VG 460 and 680)

    * Extreme Pressure (EP) mineral gear oils
      - Also extensive use in high temperature and pressure
      - Normal use of AGMA 7 and 8 viscosity grades
      - The temperature limitation just like Compounded mineral oils is around 180°F (82°C)

    * Synthetic worm gear oils
      - Major types are polyalphaolefins and polyalkelene glycols
      - Extensive application
      - No major drawbacks except costs
      - Polyalkylene glycols suffers from incompatibility with other fluids

Reference
www.gearshub.com


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