Geared transmission basics

Geared transmissions are present in sectors as diverse as automotive, aerospace, mining and ferrous metallurgy, among many others.

Below we will see what geared transmissions are, the parameters of their design, and the materials used to manufacture them.

Free download: Gear calculation: boosting efficiency in your transmissions

What are geared transmissions? Definition

Geared transmissions are mechanisms composed of two toothed wheels that mesh tooth by tooth, and when involving spur gears, they are called bull gear (the larger gear) and pinion (the smaller gear). One acts as the drive or driving gear and the other one as the driven gear.

They are widely used in precision machinery due to the accuracy they provide to a transmission when compared to other gears.

High torque at high rotational speeds is obtained thanks to the rigidity and surface hardness of various materials, among which hardened steel stands out, which is the result of a surface hardening treatment.

Geared transmissions stand out due to the accuracy of their gear ratio, which makes them perfect for precision machinery

The gear ratio in a geared transmission can be determined through the formula rt=ω2/ω1, where:

  • ω1 is the rotational speed of the drive gear
  • ω2 is the rotational speed of the driven gear
  • .The drive gear that is attached to the input shaft is called a driveshaft or ‘input’, while the driven gear that is attached to the output shaft is called the ‘output’.

Advantages of geared transmissions

Geared transmissions replace belts and pulleys for several uses, especially in industry, due to their advantages:

  • They prevent slipping. Even at high speeds, geared transmission can operate without the risk of slipping as a result of the tension generated in the mechanism. This prevents damage and accidents, in addition to making the power transmission more precise.
  • They can transmit a significant amount of power. Not only do they transmit power with great efficiency, but they can do so from the shaft of a power source to another, distant shaft. This is what takes place in internal combustion engines, for example.
  • Long service life with limited maintenance. Geared transmissions require periodic lubrication with oils to prevent the heat generated due to the friction between the teeth of both gears from deteriorating the mechanism. This maintenance is enough to obtain an optimal and prolonged performance.
  • They can be used in limited spaces. Unlike other transmission mechanisms, gears do not require significant space and can be used in small places that are difficult to access. This is especially useful in industries such as automotive and vending.

Geared transmissions prevent slipping, can transfer a significant amount of power, have a long service life with little maintenance and can be used in small spaces

Elements of geared transmissions: tooth geometry

The geometry of a gear’s tooth is determined by a series of variables, which include.

1. Circular pitch

In the pitch circle, this is the distance between homologous points corresponding to two consecutive teeth.

Pitch circle is the circle around which the gear would rotate without slipping. It is represented by a p, and can be calculated by adding the tooth’s width and the distance between two consecutive gear teeth.

2. Module

A good meshing of the bull gear and the pinion depends on this variable, m.

It is defined by the formula m=d/Z, where m is the result of dividing the pitch diameter by the number of teeth.

m = d/Z

3. Diametral pitch or dp

Expressed in inches, this is the quotient between the number of teeth and the pitch diameter.

4. Addendum Circle or Ra

The external contour reached by the teeth.

5. Root circle or Rf

This is the contour that limits the space between the teeth at their base.

6. Addendum or ha

The radial distance between the head of the tooth and the pitch circle.

7. Dedendum or hf

This is the radial distance between the pitch circle and the base of the tooth.

8. Whole depth or h

This is the sum of the dedendum and the addendum.

9. Clearance

When two teeth mesh, a free space remains that is called clearance.

Other relevant variables in a tooth’s geometry are clearance (c); working depth (hw); space width (e); tooth thickness (s); tooth face; tooth flank; flank width (b) and the pressure angle (α, also known as angle of obliquity)

Gear design parameters

The main parameters that define the gear ratio are:

– Law of gearing

The law of gearing is met when the lien of action, which is the surface where teeth mesh, crosses a fixed point within the center line at all times. More specifically:

  • It is the situation by which the imaginary line that joins both gear rotation centers passes through point O at all times.
  • This point 0 is the point of contact between both meshing teeth on a perpendicular line.
  • In addition, the length of the pitch radii of the gears and the distances from point O to the corresponding centers must coincide.

– Contact ratio

The contact ratio (ε) reflects the average teeth that remain engaged at all times.

In order to obtain an effective, low-noise operation, usually ε > 1.2

The contact ratio is the result of dividing the pressure angle by the base pitch, with the pressure angle being the sum of the angle of approach (qa), the angle of recess (qb), and the base pitch.

– Gear ratio

The gear ratio (rt) results from dividing the output speed by the input speed (rt = ω2 / ω1).

In other words, it is the ratio between the rotational speeds of both gears.

You might be interested in: Steps for calculating a gear ratio

Sometimes multi-stage geared transmissions are used. Depending on the ratio to be obtained between two shafts, a specific number of stages will be used.

How are gears manufactured?

A gear manufacturing process involves several phases. Depending on the type of device to be obtained, a series of guidelines will be followed:

  • Machining. Gear machining is the process by which gears are obtained, whether out of metallic materials, polymers or others. Milling machines and lathes prepare the gears depending on the use that they will be given.
  • Injection molding. Injection molding is used for plastic gears, which are now becoming widespread due to the advantages offered by this material. This manufacturing method yields high quality and the best precision in relation to geometric parameters.

Gear lubrication methods

During the operation of a gear, the shock force between the meshing teeth has to be reduced in order to ensure the durability of the mechanism. On the other hand, this force generates a constant friction and often leads to irregularities on the teeth’s surface.

In order to prevent this from reducing the service life of the gears and ensure that they stay operational and in top condition for as long as possible, lubrication is applied.

Furthermore, in order to prevent tooth corrosion, lubrication is used to remove heat and reduce contact tension

The main geared transmission lubrication methods are:

  • Lubricating oil dipping
  • Pressure lubrication
  • Periodic lubrication based on scheduled maintenance

Both splash lubrication and pressure lubrication take place in closed transmissions, while periodic lubrication is reserved for open transmissions.

What materials are used in gear manufacturing?

Steels, plastics, gray iron, aluminum… the materials used in gear manufacturing are many.

Within steels, we can find:

  • Forged stainless steel
  • High strength steel
  • Cast or forged aluminum alloys
  • Another set of alternatives are copper-based alloys and magnesium alloys

Steel gears, which are subsequently subject to a hardening and case hardening treatment, are used when great forces need to be transmitted and in circumstances when there is a great need for durability.

Hardened steel is one of the most widespread materials in gear manufacturing

On the other hand, the use of fireproof plastics is becoming increasingly common, including polycarbonate (PC), polyamide and PVC, as well as acetal resins.

Among non-combustible plastics, we can find polyetheretherketone PEEK, polytetrafluorethylene (PTFE) and liquid crystal polymers (LCP).

In addition, we can already find high-performance polymers such as:

  • PPS – Polyphenylene sulfur
  • PEI – Polyetherimide
  • PVDF – Polyvinylidene fluoride
  • PES – Polyethersulfone
  • PI – Polyamide
  • PPSU – Polyphenylsulfone
  • PSU – Polysulfone

Learn more: Why you should improve your safety devices using fireproof plastics

How to choose the right supplier and transmissions

Choosing the most adequate transmission will determine its successful operability. But not only that – the machinery’s performance will be affected by the type of mechanism in use.

In addition, the precision and durability of the transmission itself will depend on whether the material that has been manufactured is suitable for the conditions under it will operate.

Related reading: Tips to have in mind when buying gears

Compañia Levantina de Reductores (CLR) has a track record of over 40 years manufacturing, designing and assembling gears of the highest quality for the industry. Its significant experience in gear design and calculation for different applications makes it an industry leader.

CLR relies on the work of a professional and experienced team that is at the forefront of the latest technologies for the design, manufacturing and commercialization of gears, both in metal and plastic, which allow for the best results in electromechanical transmissions and actuators of all kinds.

Do you need assistance to choose your geared transmissions? Don’t know the types of materials that are most fitting for your sector? Do you want to address noises or vibrations in your current systems? For this and any other needs in your engineering projects, do not hesitate to contact us. We will help you find the perfect motion.

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