Rack and Pinion Steering

[jetmakerbrit]

I'm designing rack and pinion steering for a landing gear system and need some advice on gear tooth selection. Standard gear teeth proportions are unacceptable for stress reasons.

I'm working with a module 5 tooth on a 160mm PCD. I'm thinking of using an equivalent 20-degree American Standard Stub Tooth with a full radius root fillet. I would like to know if modifying the root fillet (FEA shows this to be benificial) by cutting out extra material (so as not to reduce the clearance) would cause any manufacturing problems.

I'm also slightly concerned about using the stub teeth since Machinery's Handbook says that these are "not recommended for new designs". Is there another solution I can use to strengthen my teeth? I have seen an older rack and pinion design use pinion teeth with a larger addendum than dedendum and modified rack profile but don't know how to design such a rack and pinion.

Many thanks in advance for any help provided
Jetmakerbrit

 

[diamondjim]

You can change the pressure angle to
22.5 or 25 degrees to get stronger
teeth but at the same time you reduce
the fillet radii. Using stub tooth
form you reduce your contact ratio
but they are stronger. I would ingnore
Machineries Handbook as to the caution.
I think the ideal whole depth for a
full fillet radius is 2.295 but the
standard 2.250 would be acceptable.
You can also use a long addendum
pinion and the stardard rack to get
greater strength out of the pinion.
The pinion is normally the weaker member.
The 5 Diametral Pitch or 5 Module are
nearly the same as far as strength goes.
There is also the 5/7 Fellows Stub system
that you might consider.
You may call me at 330-678-0226 if you
need anymore info.

 

[jetmakerbrit]

Thanks for the information diamondjim,

I would prefer to stick with a 20 degree pressure angle so the stub tooth sounds an acceptable way forward. I calculated the stub tooth contact ratio to be 1.389 which seems close enough to the recommended 1.4.

I understand that addendum+dedendum=whole depth but am unsure at what point the fillet should be tangential to the involute to ensure proper clearance. I was going to draw the largest radius fillet I could whilst maintaining the same point of tangency as a 0.38xModulus fillet.

Can you tell me if such a gear can be made using standard tooling?

Many thanks

 

[diamondjim]

Assuming the stub form is
.800/DP for the addendum
and 1.200/DP for the dedendum
with full fillet radius.
Note that the dedendum radius
for the stub is so close to the
1.250/DP for a full depth full
fillet radius, the vendor could
cut a full depth tooth on a stub
blank.
If the pinion is cut with a rack
hob cutter you will not have involute
interference. Since the clearance
with the stub full fillet is .400
an even larger fillet may be
developed. How many teeth are in
the pinion? If you have 14 or less
teeth, you still might want to consider
a long addendum pinion.
If you need an ACAD layout of the
pinion with the hob cutter holler.

 

[jetmakerbrit]

Thank you again diamondjim,

An ACAD (.dxf) layout of a 5 DP stub tooth with full fillet radius would be extremely helpful.

I was thinking of using module 5 teeth on a 160mm PCD giving 32 teeth over 360 degrees, since I only have teeth on 180 degrees (steering +-90) of my pinion I would have 17 usable teeth.

If I go to 5 DP stub teeth can I keep the 160mm PCD (metric required)? This gives a non-integer number of teeth but does this matter since I only have teeth over 180 deg? if this is not possible, do you know of a metric stub tooth?

Many thanks

 

[jetmakerbrit]

diamondjim
email jetmakerbrit@hotmail.com
thanks

 

[diamondjim]

There is no metric stub but they can
cut a full depth tooth on a stub
blank. They use k=.20 to identify
that the addendum is stubbed .20
times the module.
If I understand what you are doing
you are using a segment of a 32
tooth pinion with a rack?
If you want to use the 5 DP instead
you would want to cut 31 teeth on
an enlarged blank to make up the
difference in center distance.
I think a 25 percent profile shift
would give you a stronger pinion
and actually wear better if the
pinion in driving the rack.
What acad system are you using?

 

[jetmakerbrit]

The pinion teeth are being machined onto the outside of a tubular part. This part turns inside another which also holds the rack. By my understanding, a profile shift would need greater clearance. Since there is no space to grow the outer component and the rack needs to be as short as possible for stowage, I don't think profile shifting would be possible. Machining module 5 teeth with a stubbed addendum and full fillet rad seems the most likely solution. I use CATIA V5 but should be able to use igs, step, dxf or dwg files. The steering system is static for most of the time - certainly while experiencing the highest loads.

 

[diamondjim]

I sent you a layout of the tooth
space in a 1 dp layout. You will
have to scale it down to 1/5.08
to be equal to the 5 Module.

 

[jetmakerbrit]

Thanks for email - unfortunately I didn't receive the attachment. Would it be possible to send the dxf file again?
Many thanks