Parallel-axis helical gears. Image credit: How Stuff Works | Helical gears in a right angle drive. Image credit: Meccano | Perpendicular-axis helical gears. Image credit: How Stuff Works

 

 

Helical gears are cylindrical gears whose teeth are not parallel to the axis of rotation. The teeth are angled and appear as a segment of a helix. Helical gears can transmit power between parallel or right angle axes.

 

 

How Helical Gears Work

Helical gears can transmit energy between parallel or perpendicular axes through the use of helical teeth. They are designed to distribute pressure gradually along the whole tooth. Because of their tooth inclination, helical gears run smoother and quieter than other gears and are able to carry heavy loads efficiently. Due to the gradient of the teeth and the pressure applied, these gears are subject to axial thrust. This can be remedied by the use of thrust bearings and specialized lubricant.

 

Selecting Helical Gears

 

Types of Helical Gears

Helical gears are available in two teeth configurations: left handed and right hand.

  

   

Right hand | Left hand. Images credit: Agro Engineers

 

 

Helical gears transmit energy through two axial configurations: parallel and perpendicular.

 

  • In parallel-axis helical gears the two opposite-hand gears provide quiet operation and high load capacity. These helical gears must be of opposite hands.    

 Helical Gears. Video credit: ElPatillaDeCadi via Youtube

  • In perpendicular-axis helical gears, also called skew gears, two same-hand gears transmit energy through a 90° angle. The limited tooth contact of this configuration constricts work load. This type of helical alignment is comprised of two same-hand gears.

Crossed Helical Gears. Video credit: ElPatillaDeCadi via Youtube

 

 

Dimension Specifications of Helical Gears

Gears mate via teeth with very specific geometry. 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°. Helix angle is the angle at which the gear teeth are aligned compared to the axis.

         

      

 

                        

   Helix angle identified. Image Credit: Wikimedia

 

 

Selection tip: Gears must have the same pitch and pressure angle in order to mesh. Helical gears of opposite hands turn parallel drives and helical gears of the same hand turn perpendicular drives.

 

Mounting Specifications of Helical Gears

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. Helical gears can be mounted on a hub or shaft. A hub is a cylindrical projection on one or both sides of a helical 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: TT Net

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

Image credit: Direct Industry 

  • Hub Clamping Screws: The gear is attached with a screw that squeezes the inner diameter of the hub to a tight fit around the shaft.

 

 

 

 Image credit: African Tech Forum

  • 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: Precision Gear

 

Helical Gear Accessories

Due to the gradient of the gear teeth and the pressure applied to the teeth, helical gears are prone to misalignment by axial thrust. This can be remedied by the use of thrust bearings and high-pressure lubricant. If the operation cannot use thrust bearings, herringbone gears may be an alternative.

 

Helical 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.
  • 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. Helical gears' precise teeth can make them expensive
  • 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 helical gears.
  • Mounting requirements. Application may limit the gear's shaft positioning.
  • Noise limitation. Commercial applications may value a smooth, quietly meshing gear. Helical gears offer quiet operation.
  • Corrosive environments. 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, especially in helical gears where axial thrust can reposition gears during use.

Helical Gear Materials

Gear composition is determined by application, including the gear's service, rotation speed, accuracy and more.

  • 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

Boston Gear - Helical gears

 

Ashoka Gears - Helical gears

 

Agro Engineers - Helical gears

 

Wikipedia - Gears: Helical

 

How Stuff Works - Helical gears


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