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How to understand Lead Chart and Profile Chart in Helical Gear Drawing
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CheckerHater
Mechanical
This is called K-chart that gives the deviation from the true involute profile. You can see it in AGMA 390.03, daetd MAR, 1980, section 8, page 81. This is no longer the current standard but it brings this type of profile measurement however, this type or tolerance definition is not recommended by the same standard. The AGMA standards are more complicated and the information is scattered now over many AGMA standards.
PaulJackson
Automotive
popm,
The lead is the twist in a gear about its axis. A spur gear should have no twist whereas a helical has a lead (the length it takes one tooth face to twist 360 degrees.
On a lead measuring machine with a spur gear the profile stylus would traverse the tooth's length without the gear rotating... on a worm gear the stylus would traverse very little while the gear rotates a full revolution. In both when the machine is set to the specified lead a perfectly straight line on the plotted chart would indicate a zero lead error on the gear.
That said... a perfectly straight lead is not always desired... sometimes a slight crown profile is desired so that even with alignment errors between mating gears the contact profile will remain near the tooth center. There are often desired profile boundaries shown on the specification with prohibitions covering curvature reversals that would represent a concavity in the profile form. The ends of the lead profiles are sometimes have boundaries for the tooth end chamfers.
A straight Involute trace on a gear tooth would indicate that the tooth form follows a perfect involute form. If one was to take a string and wrap it around the OD of a hockey puck and tie the end of the string to the tip of a pencil then with the puck laying flat on a piece of paper... trace with the pencil on the paper a line while the string unwraps from the puck OD... this profile represent a true involute profile... the puck OD being the base circle diameter and the roll angle being the degree which the profile has unwrapped from the base circle.
That said... a perfectly straight involute profile is not often desired... oftentimes a slight crown profile is desired to provide a smooth quiet transition of contact mesh between mating gears with contact terminating prior to tooth root or tip profile transitions. There are often desired profile boundaries shown on the specification with prohibitions covering curvature reversals that would represent a concavity in the profile form. The ends of the lead profiles are sometimes have boundaries for the tooth end chamfers.
Read and apply all of the notes regarding the profile specified to verify conformance.
Paul
The lead is the twist in a gear about its axis. A spur gear should have no twist whereas a helical has a lead (the length it takes one tooth face to twist 360 degrees.
On a lead measuring machine with a spur gear the profile stylus would traverse the tooth's length without the gear rotating... on a worm gear the stylus would traverse very little while the gear rotates a full revolution. In both when the machine is set to the specified lead a perfectly straight line on the plotted chart would indicate a zero lead error on the gear.
That said... a perfectly straight lead is not always desired... sometimes a slight crown profile is desired so that even with alignment errors between mating gears the contact profile will remain near the tooth center. There are often desired profile boundaries shown on the specification with prohibitions covering curvature reversals that would represent a concavity in the profile form. The ends of the lead profiles are sometimes have boundaries for the tooth end chamfers.
A straight Involute trace on a gear tooth would indicate that the tooth form follows a perfect involute form. If one was to take a string and wrap it around the OD of a hockey puck and tie the end of the string to the tip of a pencil then with the puck laying flat on a piece of paper... trace with the pencil on the paper a line while the string unwraps from the puck OD... this profile represent a true involute profile... the puck OD being the base circle diameter and the roll angle being the degree which the profile has unwrapped from the base circle.
That said... a perfectly straight involute profile is not often desired... oftentimes a slight crown profile is desired to provide a smooth quiet transition of contact mesh between mating gears with contact terminating prior to tooth root or tip profile transitions. There are often desired profile boundaries shown on the specification with prohibitions covering curvature reversals that would represent a concavity in the profile form. The ends of the lead profiles are sometimes have boundaries for the tooth end chamfers.
Read and apply all of the notes regarding the profile specified to verify conformance.
Paul
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