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Mechanical Engineering

DrD

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Notes Reviews posted by DrD


  1. This is very short and consists entirely of terminology. There is nothing at all about bearing design, bearing stiffness, bearing heat transfer, or a host of other design concerns.

    I agree it is important to have the correct name for things, but it surely would be nice to be able to describe their characteristics mathematically.

    DrD


  2. To my understanding, item #5 is exactly backward. Cold rolled material is much more uniform that hot rolled. Hot rolling is much more similar to forging; both work the material is a hot state.

    Item #8 regarding hot rolling is not entirely correct either. If you cut a piece of hot rolled material (sawing perhaps), there is a considerable possibility of warping due to the release of internal stresses.

    Item #10 does not make sense for either hot or cold rolling. More/less stress required for what?

    I have doubts regarding item #12 for hot rolling. It is my understanding that aluminium (which is moderately reactive) is hot rolled in air.

    But then, these are just details; who really cares? Right?

    The term "cold rolling" can  be misleading. The material may start out quite cold, even as low as 32 F (0 C), but when it comes out of the cold mill, it will be far too hot to touch. This is not because heat has been applied by flame or conduction, but simply because of the massive amount of working done in the rolling process.

    Some hot rolling operations involve only a single mill stand, such as a "break down" mill that initially breaks down the ingot, or a slab mill that rolls thick slabs. A rolled section mill (for I-beams, angles, channels, etc) or a rod mill (for rebar, other rod stock) are usually hot operations. The material is cherry red as it passes through the mill rolls.

    In a cold sheet mill, the stock is received from the pickling line, after having previously passed through the hot mill. Often multiple pieces are butt welded together to make a longer piece of incoming material. This is important because once the mill is up to speed, you really do not want to have to re-start until it is absolutely essential. Material rolled at low speed, before the mill is brought fully up to speed is not the same guage or other properties as that rolled at high speed. This is because of the difference in dynamic properties of the rolling mill at different speeds.

    When we look at a picture of a rolling mill (either hot or cold), we usually see the mill stand and the ends of the rolls. The typical rolling mill is a "4-high mill" which means that there are four rolls. From the top they are (1) top backup roll, (2) top work roll, (3) bottom work roll, (4) bottom backup roll. On the cold mill where I worked many years ago, the work rolls were about 11 inches in diameter while the backup rolls were about 60 inches in diameter. This was on a mill with a width of 120 inches (10 feet). The vertical squeezing is applied to the bearings of the backup rolls, and they in turn press against the work rolls. What most do not realize is that the vertical forces is primarily there to avoid material sllp; it does not reduce the guage significantly.

    Guage reduction, that is, thickness reduction, in the rolling mill is primarily the result of stretching. Thus the first mill stand, where the stock enters initially, acts primarily as a brake, holding back the stock. Each later mill stand runs faster than the incoming stock will allow, thus stretching the material over and over as it passes through each successive stand. The interstand tension is typically about 1000 lb/in of width, so for an 80 inch wide sheet, this comes to about 80,000 lb tension between the stands. At times, the interstand tension causes the strip to break, and then all havoc breaks loose.

    DrD


  3. Most of the terminology is correct for the type of cam shown, except that the prime circle is labelled incorrectly in the drawing. But, this is just a detail, so who cares? Right?

    The displacement diagram show show that the cam angular position ranges through a full revolution for the curve shown, but again, only a detail. Right?

    The stated definition for the Pressure Angle is "It represents the steepness of the cam profile. The angle between the direction of the follower movement and the normal to the pitch curve at any point is called pressure angle." The term "steepness" is not very meaningful. Steep compared to what? This fails to mention that important fact that the pressure angle controls the side thrust on the follower shaft and thus both the bending and friction in the system.

    This post provides a lot of terminology for this particular cam type, but it gives no information at all about how to design such a system, either in terms of

    (1) system dynamics

    (2) system kinematic concerns, such as potential under cutting

    (3) contact stress considerations

    But again, those are all just details, not the sort of thing engineers are concerned about; right?

    DrD

     

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