Worm Gears / Worm Wheels
A Few Bare Facts About Worm Gears
- Archimedes is considered as the person who invented worm
gear, he used an ubiquitous screw for rotating a toothed wheel.
- A very interesting fact about the Worm gear is that the worm
easily moves the gear, but not the other way round. This is
because of the shallow angle on the worm. As soon as the gear
tries to spin it, the resulting friction between the worm and
gear keeps the worm in place.
- In latest Worm Gears a single reduction range can reach up
to 300:1
- Efficiency ranges from around 98% - lowest ratios and 20%
for highest ratios.
- Pitch line velocity up to 30 m/s is considered as ideal.
- Worm gear can facilitate a 50:1 speed reduction but not a
speed increase of 1:50.
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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 Wheel 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, worm gears can give you
1 rpm with 100 times of the torque (approx). This means that if two of these
worm wheel gears 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 And Wheel 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 Gears
efficiency
The following factors affect Worm Gears efficiency:
» Lead angle of the worm
» Installation conditions
» Sliding Speed
» Quality of the Surface
» Choice of Lubricant
Improving the Worm Gears 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.
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
