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Introduction
This article provides a detailed look at.
Read further to answer questions like:
What is a Worm Gear?
How Worm Gears are Made
Types of Worm Gears
Uses for Worm Gears
And much more …
Chapter One - What is a Worm Gear?
A worm gear is a staggered shaft gear that creates motion between shafts using threads that are cut into a cylindrical bar to provide speed reduction. The combination of a worm wheel and worm are the components of a worm gear. Speed reduction is determined by the number of worm threads and the number of teeth on the worm wheel.
The advantages of worm gears include noise and vibration reduction as well as compactness. They are made of hardened metals because of the amount of heat they generate.
Chapter Two - How Worm Gears are Made
A worm gear is the most compact type of gearing system. They are capable of being placed in very small spaces and still providing high ratio speed reduction. With proper mounting and installation, worm gear systems operate smoothly and quietly.
The typical method for manufacturing worm gears is hobbing using a hob or cutting tool; this cutting tool is similar to the gear with which the worm gear will mate. Worms can be turned, hobbed, milled, or ground.
How are Worm Gears being made
Hobbing
Hobbing is a method for producing gear teeth using a set of customized bits that are designed to make the correct pitch for the gear‘s application. Hobbing is a cold working forging process where a punch with the correct geometry is pressed into the workpiece.
The tool that is used, the punch, is referred to as a hob and is made of hardened steel that forms a helical hob cutter. The pressure and cold forging produce very accurate dimensions and excellent surface quality in the cavities.
Hobbing machines are fully automated and come in a wide variety of sizes to enable them to produce extremely small gears as well as large gears. The two parts of a hobbing machine include two spindles—one to hold the workpiece and one to hold the hob.
Milling
In the milling process, the gear is cut in a milling or jig grinder using a gear cutter and indexing or rotary table. The teeth of the gear are created by a rotating multi-edge cutter that creates teeth that are similar to the cutter. The cutter moves axially along the length of the gear tooth at the correct depth. Once the teeth are cut, the cutter withdraws, and a new blank is indexed.
The precision of the cutting depends on the accuracy of the cutter. Indexing is necessary to ensure all teeth are cut.
Grinding
Grinding uses several cutting edges at high speed and removes material at a fast rate to produce superior finishes and precise geometries. The process is ideal for hard materials; grinding of hard materials is referred to as hard finishing. A grinding machine is a multi-axes piece of equipment with bonded grinding worms called threaded wheels.
The threaded wheels are made of an abrasive material that is harder than the metal to be ground. The infeed X is set to the proper depth with a vertical feed rate Z and a lateral shifting motion Y that shifts the abrasive worm a small amount for the vertical feed rate.
Gears
The gears on a worm gear are helical; they are used to transmit power and motion between two non-intersecting shafts. There are three types of helical gear formations: spur, left hand, and right hand. The manufacture of worm gears involves left or right hand gears. The use of helical gears for worm gears is due to the non-parallel nature of the worm gear assembly.
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Chapter Three - Types of Worm Gears
One of the major benefits of worm gears is their ability to offer gear reduction and torque multiplication in a small footprint. The construction of a worm gear includes a large worm wheel that has a shallow thread and is connected to a perpendicular axes, non-intersecting gear.
For worm gears, the positioning and assembly of the teeth is critical since a very strict tooth design is required for the wheel gear and the worm gear to determine how the gears meet. Variations are dependent on how the gears touch.
The two basic types of worm gears are cylindrical or straight and cone or double enveloping. The faces of worm gears are broken into straight, hobbed, and concave. Cylindrical worms are the most common and widely used.
Worm Gears Types
In worm gear assemblies, the axial pitch of the worm must be equal to the circular pitch of the larger gear, which is the first determination during assembly. Circular pitch is the distance between the teeth points on the pitch diameter, with axial pitch being the axial distance between the points of the teeth on the worm. The threads of the worm may be left handed or right handed.
The lead refers to the distance a point on a thread travels in revolution of the worm. The lead angle is the angle that is tangent to the thread helix on the pitch of the cylinder and the plane to the axis of the worm.
Non-Throat
Non-throat worm gears do not have a throat or groove machined around the worm or worm wheel and are helical gears with a straight worm. Tooth contact is at a single point on the moving drive and both gears are non-throated. Having a single rotating point means the gear has high wear and tear. Non-throated worm gears are designed to handle small loads and are easy to make.
Single Throated
A single throated worm gear has incurvate helical teeth designed around the worm for a contact line. This type of worm gear can withstand higher power without additional wear. In this type of worm gear, only one set of threads on the worm make contact with the worm wheel. Due to the fact that there is single contact, which produces significant friction, the worm has to be much harder and stronger than the wheel.
Double Throat
The difference between single throat worm gears and double throat types is their shape. Double throat worm gears are concave on the gear and worm screw. The teeth of the gear and the shape of the worm threads are designed to increase the contact between the wheel and worm.
Double throat worm gears are able to handle high loads. The design of the double throat provides the tightest connection between the worm and the gear.
Mounting
The precise mounting of a worm gear assembly is a critical part of its use. Several points of contact are required to avoid work loads over working the lead angle.
Keyway
With keyway mounting, one or more square cutouts are made in the bore of the gear. The keyway blocks shaft rotation and helps to transfer torque.
Set Screw
A hole for a set screw is drilled into the hub. The gear is attached by inserting and tightening a set screw.
Split
A split mounting is similar to a set screw mounting but requires the use of screws and a clamp. The type of clamp varies, but all have a hole that fits over the hub of the worm gear. The hub is split or notched such that when the clamp is placed over it and tightened then separated, spread pieces are squeezed together to secure the hub.
Worm Gearboxes
A gearbox is used to change the torque or output speed of a motor. A worm gearbox has the basic design of a worm with a threaded worm gear shaft and a wheel gear. As the worm gear rotates against the worm wheel, a load is rotated by the worm wheel.
Threads
The number of threads, or starts, in a worm is the number of teeth. The speed of transmission is determined by dividing the number of teeth by the number of threads. The number of threads can be one, two, three, or more. With a one thread design, the worm gear advances one tooth. With a two thread design, the gear moves two teeth. For proper performance, the number of threads has to match the mating gear.
For extremely high helix angles, there is one thread. For smaller angles, there can be two, three, or more threads.
Chapter Four - Uses for Worm Gears
Worm gears are widely used because they can be produced in a variety of sizes and configurations. The assortment of dimensions, shapes, and designs make worm gears adaptable for any number of devices and machines.
The connection and motion of the wheel gear and worm gear produce a sliding action with low output speed and high torque. The configuration of the two gears makes for an efficient and specialized type of gearing able to perform specialized tasks.
There are two advantages to worm gears: motion transfer and single direction motion. Worm gears transfer motion at a 90o angle; this is unusual for a gear assembly. The single direction motion of worm gears prevents them from being reversed. Any attempt to change the direction of their motion will cause the worm gear to lockup and stop.
Worm Gear Usage
Noise
In applications where noise is a concern, worm gears are ideal. Unlike other gears that produce high pitched whines at high speeds, worm gears are exceptionally quiet. Worm gears are often used in public places because of their noiseless operation.
Fast Stopping
The unique design of worm gears gives them the ability to stop quickly; this makes them the right choice for the operation of elevators. Though they can‘t make up the complete braking system, they work well with other braking system components as a safety measure.
Space
The main reason worm gears are installed in many applications is the small amount of space they require. Their compact design makes them a perfect addition to applications that need efficiency but have limited space.
Shock Absorption
All worm gears are made of two different metals, with a harder metal used for the worm and a softer one used for the wheel. This design enhances a worm gear‘s ability to absorb shock loads that are found in construction equipment and other applications that have heavy loads.
Torsion
Off road and construction vehicles require different amounts of torque to be delivered to each wheel. The need for a variation in torque is due to the terrain that the wheels of the vehicles have to traverse. Worm gears enable these types of vehicles to maneuver and drive over uneven ground without the fear of damage.
Tuning
The least complicated worm gears are those that are used to tune stringed instruments. Placed on the head of the instrument, the worm gear allows for easy tuning and adjustments to raise or lower the pitch of a string.
Non-Reversible
Another aspect of worm gears in lifts or elevators is their non-reversibility, which serves as a failsafe braking system should the primary braking system fail.
Conveyor Belts
Worm gears are commonly found in conveying systems due to their single direction motion and ability to lock up when motion stops. When a conveyor shuts down, it has to remain motionless. A worm gear locks in place when motion stops and prevents the conveyor from slipping or moving forward.
Automatic Door Systems
Automatic doors have right and left movement; this has to be stopped when they have moved the necessary distance. The ending or stopping of the door‘s motion is achieved with a worn gear.
Automotive Steering Systems
In an automotive steering system, the worm gear connects to the steering wheel through the steering column. The turning of the steering wheel depends on the precise angle of the gear ratio of the worm gear system.
Chapter Five - Materials Used to Make Worm Gears
Worm gears are used for gear reduction, and they can achieve reduction ratios ranging from 20:1 to 300:1. This amount of stress placed on worm gear assemblies requires metals that are capable of withstanding the reduced speeds or increased torque. They are normally made of steel, iron, or bronze with a harder more durable metal used for the shaft of the worm.
The original worm gears were made of wood and were part of a hand crank mechanism used in shipbuilding. With the advancement of technology and metallurgy, modern worm gears are made of sturdier and more durable materials.
Materials Used to Make Worm Gears
Bronze
Bronze is typically used to make the worm wheel due to its significant mechanical properties. The metal bronze is an alloy of copper and varies depending on the other alloys combined with the copper, which can be nickel, zinc, tin, or aluminum. Typical worm wheels are made of tin bronze or aluminum bronze due to their strength and resistance to fatigue, friction, and wear.
Brass
Much like bronze, brass is used to make the wheel gear; it is a softer metal and prevents wear on the shaft of the worm gear. The brass worm gear configuration is used for light loads since brass can only endure lower load amounts.
Steel
Though there are steel worm gears and worm wheels, the usual configuration is a steel worm gear with a bronze or brass worm wheel. Steel is used for its durability, tensile strength, and longevity. The steel worm gear and steel worm wheel combination are more expensive and require extensive time and effort to repair when they fail.
Plastic
Plastic worm gears are used for extremely light loads such as automotive components and robotics. When paired with metal worms, they have a quieter performance and do not require lubrication. Plastic worm gears are lightweight and resistant to corrosion and chemicals.
Stainless Steel
Stainless steel worm gears are made of 303 and 316 grade stainless steel. They are ideal for damp and wet conditions since they do not rust or corrode. They are normally used in conditions where cleanliness is a necessity such as food and beverage manufacturing. They are easy to clean and have smooth surfaces and exceptional durability.
Chapter Six - Worm Gear Lubrication
The amount of stress, torque, and motion worm gears deal with requires the use of lubricants to provide good lubricity in metal to metal contact. Mineral based lubricants are commonly used with worm gears; they are made from mineral oil, a product of crude oil. The goal of a worm gear lubricant is to protect the worm drive from friction, corrosion, and inefficiency.
It is impossible for a lubricant to prevent gear wear indefinitely. A combination of natural and synthetic additives provide extra protection and increase the worm gear‘s longevity.
The viscosity of a lubricant prevents the worm from touching the wheel in the worm gear assembly. The load and size of the gearing determines the type of lubricant.
There are difficulties associated with lubricating worm gears due to their design. Much of the problem is caused by the sliding motion of the gears as it wipes away the lubricant.
The sliding motion of worm gears is the reason for the use of metals with a low friction coefficient; the worm wheel is a yellow metal such as bronze or brass and the worm gear is a hardened metal such as steel.
Worm Gear Lubrication
Compounded
Compounded oil has a mineral base that has rust and oxidation inhibitors blended with four to six percent acidless tallow or synthetic fatty acid. They have the ability to adhere to cylinder walls with a temperature limitation of 180° F (82° C). The additives form a protective barrier between the contacting surfaces when conditions go beyond the bulk oil's film strength.
The viscosity of the lubricant depends on the worm‘s size, type, speed, and operating conditions. American Gear Manufacturer Association (AGMA) compounded oils classes 7 and 8 are normally used.
Extreme Pressure (EP)
In conditions of high pressure and temperature, an EP oil reacts with a metal surface to form a chemical layer that prevents wear and welding. EP lubricants work well in conditions where there is shock or vibration and are ideal protection of steel. EP gear oils have the same temperature restrictions as compounded oils and are AGMA grades 7 and 8.
Synthetic
Two types of synthetic gear oils have found success with worm gear applications: polyalphaolefin (PAO) and polyalkylene glycol (PAG). PAO and PAG have excellent lubricity qualities and characteristics.
PAO is adaptable to low and high temperature applications and is compatible with mineral oils. Unlike typical synthetics, PAO does not attack seals or paint and has an anti-wear mineral added to improve its boundaries. The only disadvantage of PAO is its cost.
PAG has the same capabilities as PAO with a higher viscosity index than other synthetics. It has anti-wear properties but no EP additives. PAG does not interact well with other fluids and does attack paints and seals.
Conclusion
A worm gear is a staggered shaft gear that creates motion between shafts using threads that are cut into a cylindrical bar to provide speed reduction.
The advantages of worm gears include noise and vibration reduction as well as compactness.
A worm gear system is the most compact type of gearing system; this makes it capable of being placed in very small spaces and still providing high ratio speed reduction.
One of the major benefits of worm gears is their ability to offer gear reduction and torque multiplication in a small footprint.
The assortment of dimensions, shapes, and designs make worm gears adaptable for any number of devices and machines.
Leading Manufacturers and Suppliers
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