Worm gear Image credit: How Stuff Works | Worm gear reducer Image credit: Chinabase | Enveloped worm gear Image credit: Poona Gear

 

Worms and worm gears are gear sets that offer high gear reduction and torque multiplication with a small footprint. A worm drive is a cylindrical gear with a shallow spiral thread that engages the worm gear in a non-intersecting, perpendicular axes configuration.

 

           Worm Gear Speed Reducer Video credit: 859m via Youtube

 

How Worms and Worm Gears Work

Efficiency of a worm drive is determined by the lead angle and the number of threads in contact with the worm gear.  A high lead angle on the drive reduces frictional losses and heat. A low lead angle reduces gear speed while proportionally increasing torque.


Friction is an issue with all worm sets; the worm gear cannot transfer motion back to the worm drive in most instances. Lubrication and ground teeth both contribute to the sets' overall silence while minimizing friction. Worm gear sets are usually produced in pairs due to their precision.

 

Types of Worms and Worm Gears

Variance amongst worm gear sets is generally constricted to handedness (left or right) and the concavity available to worm drives and worm gears along gear widths. Their use depends on the mechanical efficiency required.

 

  • Non-enveloping worm gear sets do not have concave features and the straight plane of contact between gears places the highest level of stress on the gear teeth.  

            Image credit: Wikimedia

  • Single-envelop worm gear sets contain a worm gear with a concave tooth width, allowing the worm drive to nestle into the gear and increasing efficiency.

                Image credit: Stepan Lunin

  • Double-envelop worm gear sets contain both a word gear with a concave tooth width, and a worm drive with a concave profile. This design maximizes efficiency.

              Image credit: Stepan Lunin

 

 

Dimension Specifications of Worms and Worm Gears

Gears mate via teeth with very specific geometry. Pitch is a measure of tooth spacing and is expressed in several ways.

  • Diametral pitch (DP) is the ratio of the number of teeth to the pitch diameter of a gear; a higher DP therefore indicates finer tooth spacing. It is easily calculated by the formula DP= (N+2) ÷ OD, where N is the number of teeth, and OD represents the circumferential measurement.
  • Circular pitch (CP) is a direct measurement of the distance from one tooth center to the adjacent tooth center. In a worm drive, it is called axial pitch and can be measured by the formula  CP= Π ÷ DP.
  • Module (M) is a typical gear discipline and is a measurement of the size and teeth number of the gear. Gears measured in inches earn 'English module' distinction to prevent confusion. M = OD ÷ N
  • Pressure angle is the angle of tooth drive action, or the angle between the line of force between meshing teeth and the tangent to the pitch circle at the point of mesh. Typical pressure angles are 14.5° or 20°.
  • Lead angle is the angle at which the gear teeth are aligned compared to the axis. This is also the point of contact between the drive and the gear. This is also known as helix angle.

 

Image credit: Access Science

 

 

Selection tip: Worm drives and worm wheels must have the same pitch and pressure angle in order to mesh, but may not mesh if the envelope concavity is mismatched.

 

 

Mounting Specifications of Worms and Worm Gears

The mounting of worm gears is critical to their implementation. Multiple points of contact are necessary between the drive and gear so high work loads do not overwork the same lead angle, which could lead to gear failure. Enveloped worm gear sets are normally assembled in the same housing, to ensure proper mating and due to the sets' small footprint.

 

Consider the gear center, bore diameter and shaft diameter. The gear center can be a bored hole or an integral shaft. The bore diameter is the diameter of the center hole. The shaft diameter is the diameter of the shaft for gears with an integral shaft. Worms and worm gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a worm or worm gear, often for the provision of a screw or other shaft attachment mechanism. Hubless gears are typically attached via press fit, adhesive or internal keyway.

 

Shaft mounting choices include the following:

  • Keyway: One or more square cutouts exist in the gear bore for exact mounting on the shaft. 

 

Image credit: Boston Gear

  • Set Screw: The gear is attached to the shaft by screws through the hub.

 

Image credit: Direct Industry

  • Split: The hub is split into several pieces that are tightened down by a separate clamp to grip the shaft. 

 

Image credit: SDP/SI

  •  Simple Bore: A straight bore designed for adhesive attachment.

 

Image credit: Boston Gear

 

 

Worm and Worm Gear Applications

Application requirements should be considered with the workload and environment of the gear set in mind. 

  • Power, velocity and torque consistency and output peaks of the gear drive so the gear meets mechanical requirements. Worm gear sets are especially suitable for high torque and gear reduction applications.
  • Inertia of the gear through acceleration and deceleration. Heavier gears can be harder to stop or reverse.
  • Precision requirement of gear, including gear pitch, shaft diameter, pressure angle and tooth layout. Worm gear sets have precision meshing.  
  • Handedness (left or right teeth angles) depending the drive angle.
  • Gear lubrication requirements. Some gears require lubrication for smooth, temperate operation and this is especially true for worm drives, which may also employ ground teeth to reduce friction.
  • Mounting requirements. Application may limit the gear's shaft positioning.
  • Noise limitation. Commercial applications may value a smooth, quietly meshing gear. Worm gears offer the quietest operation.
  • Corrosion resistance. Gears exposed to weather or chemicals should be especially hardened or protected.
  • Temperature exposure. Some gears may warp or become brittle in the face of extreme temperatures.
  • Vibration and shock resistance. Heavy machine loads or backlash, the deliberate surplus space in the circular pitch, may jostle gearing.

  • Operation disruption resistance. It may be necessary for some gear sets to function despite missing teeth or misalignment. If mounted correctly, most worm drives will operate if missing a tooth.

Worm and Worm Gear Materials

Worm gear sets are usually used to reduce speed and increase torque. Since the worm drive undergoes more contact stress cycles than the worm gear, the worm drive is usually of a stronger material.

  • Cast iron provides durability and ease of manufacture.
  • Alloy steel provides superior durability and corrosion resistance. Minerals may be added to the alloy to further harden the gear.
  • Cast steel provides easier fabrication, strong working loads and vibration resistance.
  • Carbon steels are inexpensive and strong, but are susceptible to corrosion.
  • Aluminum is used when low gear inertia with some resiliency is required.
  • Brass is inexpensive, easy to mold and corrosion resistant.
  • Copper is easily shaped, conductive and corrosion resistant. The gear's strength would increase if bronzed.
  • Plastic is inexpensive, corrosion resistant, quiet operationally and can overcome missing teeth or misalignment. Plastic is less robust than metal and is vulnerable to temperature changes and chemical corrosion. Acetal, delrin, nylon, and polycarbonate plastics are common.

  • Other material types like wood may be suitable for individual applications.

Resources

 

Design Aerospace LLC - Gearing Arrangements

 

Rush Gears - Worm Gears

 

Roy Mech - Worm Gears

 

Gear Solutions - Envelope Worm Gears

 

Poona Gear Industries - Worm Repair Gears


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