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Thursday, 21 February 2013

Gear design software

Following from the last post the following list of sites offer Gear profile generators to create the gears for you.

This first Gear template can be used either on line or you can purchase a fuller more feature rich version for use on your PC. The output is to an HPGL format that can be used for direct printing .
One of the benefits of the paid for version is that it can export to DXF format

This software will provide templates for all manner of gears, ratchets and clock escapements along with the facility to adjust the profiles. Output as DXF files. This is not a free program but you can download a demo version to try it

eMachineShop is a Free 2D drawing package with a module for drawing gears, easy to use, you simply add the basic gear parameters and it draws the gear which can then be exported as a DXF file.
The program is free but unlike the first two programs it lacks the facility to see to the main gear and the pinion on-screen at the same time.
Simple Free program , input gear parameters and then save as DXF.

This is a program to generate tooth profiles in an epicyclic form rather than the more generally used involute form. For a description of the difference see the following:-

Comparison Between Involute and Cycloidal Gears
The output from this program is in DXF format and can be viewed in a browser.

Wednesday, 20 February 2013

Gear Train design for the wooden clock

The gears used in all of my clocks have geometry based on the standard gear profile formulae with some adjustment to the tooth profile to thin them down a little to make it a bit less sensitive to the inaccuracies inherent when using hand cutting methods to produce them.
When designing clock gears I generally use the metric system to define the teeth but both metric and imperial can be used interchangeably. The chart shown below shows the formulae I use for gear calculation.

For the gear teeth to engage with each other they must both be calculated using either the same Diameter Pitch (DP) or the same Module these two terms relate to the way the teeth are spaced around the Pitch Circle Diameter (PCD). You can see from the chart below that as the Mod increases so does the size of the tooth, and visa versa for the DP.
If you are working with metric dimensions you will use the MOD and if you are working in imperial dimensions you will use the DP.
Generally speaking, if you are building a wooden clock then you will be using Mod 1.5 or above.

Once you have decided on the DP or Module you are going to use that to calculate all the other features using the formulae in the chart at the top. Both these charts come from the Technical section of the HPC Gears catalogue 
There are two main gear trains in a clock, the minutes train that requires a total ratio of 60:1 this runs between the shaft carrying the minute hand and the escapement wheel. The escape wheel normally turns once a minute and it connects through the train with the shaft holding the minute hand, which turns once an hour, hence the 60:1 ratio.
The second train runs between the minute hand and the hour hand which requires  12: reduction. This ratio normally has to 2 sets of gears with ratio's of 3:1 and 4:1 and the centre distance of each are normally the same as the gears are mounted on the same pairs of shafts, I always use a 8 teeth and 32 teeth pair along with a 10 teeth and 30 teeth pair, as it allows the shafts to be shared.
With the 60:1 ratio there are many more options open to you and will to a large extent depend on the design you are trying to achieve, so I must leave that up to you to decide. Having said that a simple set of 3 gears using 3 pair sets with ratio's of 3:1 4;1 and 5:1 works quite well as when multiplied together they will give you a 60:1 ratio.
Having decided on the  gear arrangement I normally try to determine the Module value I am going to use and that relates to the size of the gears I want. To determine that I would normally work out on CAD or on paper what size I want the largest gear to be, so in my case this is usually a 60 tooth gear. As an example I have decided the gear needs to be about 125mm (5ins) diameter so from the chart above I can work out a value for the module.
Module = Outside Diameter mm / ( Number of teeth +2)
This gives a value for the Module of 2.016 so round that down to 2.0 and you can now use that value to calculate the sizes for all of your gears.
If you are an organised person you would use a spread sheet to enter all your values to work out the relevant gear sizes, I never got round to doing this so finish up working them out on a calculator.
The chart below shows a typical gear pairing for a 3:1 gear ratio and a Module of 2 using a 60 tooth and a 20 tooth gears. The tooth thickness is slightly less than the normal 50% of the CP value I have used between 44% and 48% of that value depending on what looks to work best in the CAD simulations.