Magnesium transmission casings 2

[Billsy]

Does anyone know what the benefits of magnesium transmission casing are; structural/mechanical properties as well as cost compared to Al/Fe? Examples of applications would be helpful.

 

[CoryPad]

Higher density-normalized strength and stiffness, and higher damping capacity. This allows lighter cases that transmit less NVH.

All new Volkswagen transmission cases are made with magnesium alloys.

You can find general information using Google (

http://www.google.com)

and keywords Volkswagen magnesium transmission.

You can find technical information using Scirus (

http://www.scirus.com)

and keywords Volkswagen magnesium transmission.



Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tbuelna]

Billsy:

There is no real benefit to Mg transmission casings. Although the metal has a lower density than aluminum, aluminum is better with respect to specific stiffness (the ratio of stiffness to density).

The downside to Mg casings are that they are more susceptible to corrosion. Magnesium also gives more problems when trying to maintain dimensional relationships with steel gears/shafts/bearings, during temperature change. Mg has a CTE about 25% greater than Al.

As far as cost, cast magnesium is not much more expensive than cast aluminum. It is a relatively plentiful element (I believe high quality Mg is extracted from dolomite).

If you wish to examine their properties more closely, you can go to the link below. The typical aluminum sand casting alloy is A356 or A357. A typical magnesium sand casting alloy would be AZ92 or ZE41.

http://www.matweb.com/index.asp?ckck=1

Good Luck.
Terry

 

[CoryPad]

Terry,

There are real benefits to using magnesium alloys for transmission cases. I was imprecise in my first post - Al alloys have higher E/&[ignore]rho[/ignore];, but transmission cases are bending critical components, and therefore the property index is either E[sup]0.5[/sup]/&[ignore]rho[/ignore]; or E[sup]0.33[/sup]/&[ignore]rho[/ignore];. Mg alloys are superior under these conditions.

Automotive transmission cases are made using high pressure die casting, not sand casting. Therefore, suitable alloys are AZ91D and A380. Using Matweb for properties, one can calculate the following:

[tt]
Property AZ91D A380
-------- ----- -----
E 44.8 71
&[ignore]rho[/ignore]; 1.81 2.76
E/&[ignore]rho[/ignore]; 24.75 25.72
E[sup]0.5[/sup]/&[ignore]rho[/ignore]; 3.70 3.05
E[sup]0.33[/sup]/&[ignore]rho[/ignore]; 1.96 1.50
[/tt]
Mg alloys do have larger thermal expansion, and the cost can be higher (~10%-50% depending on design, volume, etc.). Dolomite is one of the major ores from which to obtain magnesium metal.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[swall]

The corrosion issues with magnesium can be mitigated.In the U.S., there has been at least 15 years of satisfactory usage of magnesium transfer case housings on 4wd light trucks and SUV's.The key is to avoid galvanic corrosion issues by isolating the mag housings from aluminum or iron components by the use of special gaskets.

 

[patprimmer]

I think the trusty old VW beetle, Kombi and Type 111 all had magnesium transaxle housings, and even crankcases at times. They seem to last 30 or 40 years, in a wide variety of environments, without corrosion problems

Regards
pat

 

[GregLocock]

The trick is to keep them away from salt water. I evaluated painted magnesium for one product. We dropped samples of each system into standard seawater. The magnesium parts were like an old advert for Alka Seltzer - plink, plink, fizz.



Cheers

Greg Locock

 

[Billsy]

Thanks to everyone who has contributed here. This is the first time I've used Eng-Tips and have been amazed by the response. Thanks again and keep up the good work.

 

[coughdrop]

Magnesium high purity alloys such as AZ91 derivatives are very suitable for trans' cases which are made using the High Pressure Diecasting process. VW as quoted above have extensive experience. Corrosion is less of an issue with the modern alloys but, care must be taken with insertion of other metals used for fixings etc.. Meridian is a North American company (with a plant in Nottingham in the UK) which has many years of experience manufacturing Automotive application Mg castings. NADCA will also be a source of examples/case histories.
To get the best from this material, please remember to design the Trans' case as Mg, not convert a current Al design. This should give a comparable cost to Al.
Good luck,
John.