Worm Gear Calculation

No one that is involved in design wants to deal with Worm gears. Just the engineers or designers that are familiar with this type of applications have no problems to understand the development, working conditions, manufacturing and design of a worm gear set.
In my case, it took me a long period of time to have an idea about the minor details of this mechanical element, and I am just able now to define them geometrically, calculate the forces and evaluate the proper assembly and understand some cases of failures.
I have read some articles and gear handbooks to verify the informatoin given in this application for Android, wich is going to be available in three of four weeks.

Here you have some links to useful information:

KHK gears
Autodesk Basic Information
Research gate Worm Gears PDF

I will add more information of worm gears while I am progressing with the application.

Worm Input motion:

Worm in YouTube

Here we have the worm wheel in motion with the worm. Input and output together.

Now the final animation is ready. Time to work in the app.

It took me more time than I wanted, but the app is now available in Google Play.
The application will help you to design a worm gear according to the reccomendations found in different articles and books.
You may need to know the transmission ratio of your gearset, as well as the center distance between worm and wormgear.
After filling those two variables, the worm should be defined with a set of parameters that are neccesary to calculate the whole gearset. The app will give you some indications to fill them.

The following images explain the way to work with this new application:

The application is available in Google Play:

What have I learned during the application development and the usage of this type of mechanical ellements...
First of all, this type of gear set has the difficulties of a typical gear set design with the difficulties of the a bevel gear transmission.
The capacity of provide a huge reduction in a very small space would drive engineers to think in this transmission as a solution instead of using, maybe a planetary system. But, consider the following facts:

- Worm gear manufacturing is difficult and should be done by specific manufacturing suppliers that could provide the quality you require for your application.

- Worm wheel, may not be as difficult to machine as the worm and it will not need a quality grade as tight as the worm also, because it will tend to wear and accomodate the worm geometry to the tooth surface and part of the geometric deviations will dissapear, but it will need some work to think the way you want to attach this worm wheel (in general a piece that would be serviceable) to the ouput shaft.

- The location of the gear contact has, in worm gear design, three possible linear deviations. Compared to a typical spur gear set, the worm and worm wheel could move in X, Y and Z direction, Probably the center distance is the one that in general can not be adjusted by shims, but the axial location of the worm as well as the axial direction of the worm wheel can be adjusted slightly.
Doing that, you can reduce the transmission error and adjust the backlash of your system.

- Lubrication, Lubrication and Lubrication. It is essential to provide good lubrication to the gear mesh because the relative motion between gears is almost sliding. Therefore, oil type and oil lubrication method should be designed carefully.

Here you have some links to useful information I found about worm, and worm wheels:

• Nptel Worm Gears
• khkgears.net
• Worm Gear Design, 
• TY  - CHAP, AU  - Suleiman Khayal, Osama, PY  - 2017/07/20, SP  -, T1  - Worm GearsER  - 
• Knowledge.autodesk.com
• www.copperalloys.net
• www.machinerylubrication.com
• Lubricate-worm-gears

Shaft - Hub Press Fit Calculator

When I started to work in things related with mechanisms, materials, engineering details, transmissions...etc I was struck by the torque and resistance capacity of these type of press fit joints. 

Nobody told me, by this time, how well should be controlled the dimensions of the parts that create the joint. Even small tolerances expand considerably the difference between min and max torque capacity.

I hope the list of suggestions I am writing here help you to design your joint:

• If your joint rotates and you can add a key in your joint, just do it, It is a safety device that will help you to sleep better.
• Double check the expansion of the external part, the hub. Only in the case your surrounding parts are too close to the hub.
• Provide generous chamfer at the hub and shaft entries.In general a 20 degree angle is enough. During the assembly, try to avoid an edge contact between parts, so look at the interference sequence and verify that chamfer ends do not coincide by the time you start to press. Sometimes a difference in the chamfer angle helps.
• Tolerance ranges capable and reliable for a standard grinding are around ±0.013, Eventhough you can use a tight values like ±0.008 but probably your cpk would be far away from 1.33 unless you pay so much for the parts.
• Take care with thinner parts. On shafts, the inner diemater can end as an oval. If you press fit so much a bearing cup, the rolling ellements could find areas with less clearance than others and that would generate a speed reduction in your rolling elements, sliding, heat and infant failures. In the other hand, a thin hub, will copy so well the shaft geometry that any size imperfection will end at the outside of the hub. Also the torque capacity will be very poor and depending on the material, the elastic curve of the hub can easily be overpassed and finally yields to a permament deformation.
• Use if possible a retainer compound,
• Assemblymag.com
• Loctite Retainers
• If possible, use the same material for hub and for shaft. If not possible, at least both would have the same elasticity module. In case using a different material have a look at the part with the lowest module, this one will limit the capacity and the geometry of the joint.
• To obtain better results and to avoid excessive wear during the assembly, due to the friction between surfaces a hub preheat or a shaft temperature reduction is needed in some cases. Also that effect will help the assembly process o reduce the force needed to join the parts. Caution must be taken to avoid preheat at temperatures that could temper the parts and reduce its surface hardness. In general, I do not accept prehteats that exceeds 180 ºC.

Machining Tolerances

This application help designers and engineers while designing the parts for any type of mechanical device.
The application will give you a tolerance range, equal bilateral, for a especific manufacturing process selected. Turning, Grinding, Broaching, Reaming... up to 12 different types.

  Machining Tolerances.

As a designer of mechanical components, here are a couple of recommendations I do always consider when assigning tolerances:

• It's very important to define a correct dimension composition in your drawing. Avoid having accumulation of lineal dimensions. They will only add variation in your part and in your assembly.
• Tolerance values below ±0.013 are difficult to maintain under regular conditions, even when talking about bearing bores or bearing supports, requesting values below those 0.013 mm tends to be a problem for manufacturing and quality. If you can live with something avove it, your supplier or manufacturing collegue will appreciate it.
• Face to face location in case of bearings supports (shafts or pinion gears) could be in values around ±0.05 mm. 
• Think always in those features that would need to be machined after heat treatment. In general, it is common to use a grinding operation to compensate deformation and to obtain the desired roughness. Depending on the size of the feature, values below ±0.013 can be achieved adding more cost to the part.(again, cpk requirements could be an issue for quality and for the supplier)
• Try to not assign geometric tolerance values lager than feature tolerances, so think twice about the values you define in your parts.
• Use equal bilateral tolerances in your drawings. It will help you with your stackup calculations (comming soon application) and if you are using 3D models, and your manufacturing does also, your parts will be machined centered always.
• Cost of the tolerances:

• General Tolerance values: This this the tolerance table intended to simplify drawing indications and specifies general tolerances in four tolerance classes. It applies to the dimensions of work pieces that are produced by metal removal or are formed from sheet metal. It contains three tables and an informative annex with regard to concepts behind general tolerancing of dimensions. 

         General tolerances for linear measures according to DIN ISO 2768-1

         Table one - Permissible deviations for linear dimensions except for broken    

         edges (external radius and chamfer heights) 

Designation	Description	Permissible deviations for basic size range in mm
		from 0.5# upto 3	Over 3 upto 6	over 6 upto 30	over 30 upto 120	over 120 upto 400	over 400 upto 1000	over 1000 upto 2000	over 2000 upto 4000
f	fine	±0,05	±0,05	±0,10	±0,15	±0,20	±0,30	±0,50	Nil
m	medium	±0,10	±0,10	±0,20	±0,30	±0,50	±0,80	±1.20	±2.00
c	coarse	±0,20	±0,30	±0,50	±0,80	±1.20	±2.00	±3.00	±4.00
v	very coarse	Nil	±0,50	±1.00	±1.50	±2.50	±4.00	±6.00	±8.00

Table two - Permissible deviations for broken edges (external radius and chamfer heights)

		Permissible deviations for basic size range in mm
Designation	Description	from 0.5# upto 3	over 3 upto 6		over 6
f	fine	±0,20	±0,50		±1.00
m	medium
c	coarse	±0,40	±1.00		±2.00
v	very coarse

Table three - Permissible deviations for angular dimensions

		Permissible deviations for ranges of Lengths in mm of shorter side of the angle concerned
Designation	Description	upto 10	over 10 upto 50	over 50 upto 120	over 120 upto 400	over 400
f	fine	±1.00°	±0.500°	±0.333°	±0.166°	±0.083°
m	medium
c	coarse	±1.500°	±1.000°	±0.500°	±0.250°	±0.166°
v	very coarse	±3.00°	±2.000°	±1.000°	±0.500°	±0.333

General tolerances for form and position DIN ISO 2768-2 is for simplifying drawing and fixes general tolerances in three tolerance classes for form and position. By choosing a special tolerance class exactly the precision level common in workshops should be taken into account.

General tolerances for straightness and evenness in mm

		Permissible deviations for basic size range in mm
Designation	Description	upto 10	over 10 upto 30	over 30 upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.02	0.05	0.10	0.20	0.30	0.40
K		0.05	0.10	0.20	0.40	0.60	0.80
L		0.10	0.20	0.40	0.80	1.20	1.60

General tolerances for straightness and evenness in mm

		Permissible deviations for basic size range in mm
Designation	Description	upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.20	0.30	0.40	0.50
K		0.40	0.60	0.80	1.00
L		0.60	1.00	1.50	2.00

General tolerances for symmetry

		Permissible deviations for basic size range in mm
Designation	Description	upto 100	over 100 upto 300	over 300 upto 1000	over 1000 upto 3000
H		0.50
K		0.6-		0.80	1.00
L		0.60	1.00	1.50	2.00

Source : www.huaxing.com
Links to Information:

•  Typical Tolerances
•  Machining Tolerances
•  Cost of Tolerances