Magnesium transmission casings
Magnesium transmission casings
(OP)
Does anyone know if magnesium casings would have lower NVH characteristics than aluminium.
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
Magnesium transmission casings
|
Magnesium transmission casingsMagnesium transmission casings(OP)
Does anyone know if magnesium casings would have lower NVH characteristics than aluminium.
Red Flag SubmittedThank you for helping keep Eng-Tips Forums free from inappropriate posts. Reply To This ThreadPosting in the Eng-Tips forums is a member-only feature.Click Here to join Eng-Tips and talk with other members! |
ResourcesWhat is rapid injection molding? For engineers working with tight product design timelines, rapid injection molding can be a critical tool for prototyping and testing functional models. Download Now
The world has changed considerably since the 1980s, when CAD first started displacing drafting tables. Download Now
Prototyping has always been a critical part of product development. Download Now
As the cloud is increasingly adopted for product development, questions remain as to just how cloud software tools compare to on-premise solutions. Download Now
|
RE: Magnesium transmission casings
However, E/rho and sigma/rho are quite similar for both materials, so you tend to have to use the same weight of magnesium as aluminium, and end up with similar modal properties.
There is an additional complexity at higher frequencies. Panels have a coincident frequency where the speed of bending waves in the panel matches the speed of sound in air. This gives perfect coupling of vibration to sound. I suspect that magnesium will have a diferent coincident frequency to aluminium, and depending on the gear teeth numbers this may make your radiated gear whine problems different.
I have always argued against using magnesium for gearboxes in RWD cars because they do it to save weight, so the engine/gearbox assembly gets less stiff, so the bending frequency drops, and that bending frequency is the cause of all sorts of NVH issues.
Cheers
Greg Locock
RE: Magnesium transmission casings
Most complex components like automotive transmission housings have equally complex loading, with a great deal of bending involved, both in the form of global bending of the entire housing, and local bending/buckling of a particular section. When global bending is involved, the beam mode becomes important, i.e. external or self-weight loading, stiffness and length specified, section area is free. For this case, the Performance Index (term used by M. F. Ashby in his book Materials Selection in Mechanical Design, becomes E1/2 / rho or sigma1/2 / rho. This favors Mg over Al due to its much lower density.
When plate bending or buckling is important, i.e., loaded externally or by self weight in bending, stiffness, length, and width specified, thickness free, the Performance Index becomes E1/3 / rho or sigma1/3 / rho, which favors Mg even more than the previous case.
Another factor to keep in mind are the minimum practical wall thickness that can be achieved during casting. In pressure die casting, significantly thinner walls can be achieved with Mg than in Al. This becomes important when the loads are small enough that the Al wall thickness cannot be made any thinner due to manufacturing/casting constraints, and a similar wall thickness of Mg will perform adequately, and hence weigh less.
RE: Magnesium transmission casings
The weight of a complete gearbox is around 50-80 kg. The weight of the casing alone is 10-20 kg. Therefore a formula based on the self-weight of the casing is going to be very misleading.
Cheers
Greg Locock
RE: Magnesium transmission casings
RE: Magnesium transmission casings
section area (for beam-type bending)
thickness (for plate bending or buckling)
The idea of the Performance Index does not mean that you would only consider the 10-20 kg of the casing and ignore the mass of the rest of the gearbox/transmission. What it does is allow for designing around the most efficient combination of material, shape, etc.
RE: Magnesium transmission casings
AZ91D Mg A830 Al
density .065 lb/in3 .097 lb/in3
ult TS 34,000 lb/in2 48,000 lb/in2
yld TS 23,000 lb/in2 24,000 lb/in2
E (tension) 6.5x10^6 lb/in2 10.3x10^6 lb/in2
CTE 26 min/in/degC 22 min/in/degC
specific damping
capacity
@5000psi 25% 1%
@15,000psi 53% 4%
There is also the significant issue of corrosion that must be addressed when considering the use of magnesium in an automotive underbody area.
I hope this information is helpful.
Terry
RE: Magnesium transmission casings
You are right. The curve of system mass vs stiffness will be offset by the dead load, but the shape of the curve will be the same.
By the way, passenger car gearbox casings for rear wheel drives are stiffness limited (except for some details near the fasteners) so magnesium's greater stress/rho is not especially helpful. Current best practice is to cast the bellhousing, trans, and extension housing, in one piece, to get rid of the surprisingly high compliance of the bolted joints. I have seen a paper suggesting that a structure that consists of thin walls and ribs is noisier than a thicker constant section of the same weight. I suspect this is a matter of details.
Cheers
Greg Locock