learning with purpose march 25, 2013 learning with purpose march 25, 2013 22.322 mechanical design...

Learning with Purpose March 25, 2013Learning with Purpose March 25, 2013

22.322 Mechanical Design II

Spring 2013

Learning with Purpose March 25, 2013

The requirement for static balance is that the sum of all forces on the moving system must be zero• Unbalanced forces of concern are due to the accelerations of

masses in the systemAnother name for static balance is single-plane balance• The masses generating the inertia forces are in, or nearly in,

the same plane• Essentially a 2-D problem

Devices that can be statically balanced are all short in the axial direction compared to the radial direction• Can be considered to exist in a single plane

Lecture 20

Some Review

Note that for balancing, it does not matter what external forces are acting on the system the only forces acting on this system are inertia forces

External forces cannot be balanced by making any changes to the system’s internal geometry

For balancing, it does not matter how fast the system is rotating, only that it is rotating.

In Lecture 19, we figured out what

mbRb needed to be to statically

balance the links


Dynamic balance is sometimes called two-plane balance• Requires that the sum of the forces AND sum of the

moments must be zeroDevices that can be dynamically balanced are all long in the axial direction compared to the radial directionIt’s possible for an object to be statically balanced but not dynamically balanced.

Lecture 20

Some Review

Statically balanced Unbalanced moment

Inertia forces form a couple which rotates with the masses about the shaftRocking couple causes a moment causing the left and right ends of the shaft to lift and drop


To correct dynamic imbalance requires either adding or removing the right amount of mass at the proper angular locations in two correction planes separated by some distance along the shaft.• Creates the necessary counterforces to statically balance the system

and also provide a countercouple to cancel the unbalanced moment• Automobile tire the two correction planes are the inner and outer

edges of the wheel rim• Correction weights are added at the proper locations in each of these

correction planes based on a measurement of the dynamic forces generated by the unbalanced, spinning wheel.

Lecture 20

Some Review


Complete balance of any mechanism can be obtained by creating a second “mirror image” mechanism connected to it so as to cancel all dynamic forces and moments.This approach is expensive and only justified if the added mechanism serves some purpose (increasing power).

Lecture 20

Balancing LinkagesEssentially adding a “dummy” mechanism just to cancel dynamic effects

Certain configurations of multicylinder internal combustion engines do this pistons and cranks of some cylinders cancel inertial effects of others.


Many methods have been devised to balance linkagesSome achieve a complete balance of one dynamic factor (shaking force), at the expense of other factors (shaking moment)

• Shaking force – net unbalanced force acting on mechanism• Shaking moment – net unbalanced moment acting on mechanism

Others seek an arrangement that minimizes (but does not zero) shaking forces, moments, and torques for a best compromiseFor a four bar linkage, the rotating links (crank and rocker) can be individually statically balanced using the balancing methods we described in lectures 18 and 19.Note that the process of statically balancing a rotating link, in effect, forces its mass center (CG) to be at its fixed pivot and thus stationary.But the coupler (which can have complex motion) has no fixed pivot and thus its mass center is, in general, always in motion.

Lecture 20

Balancing Linkages


Lecture 20


Lecture 20

Essentially zero (computational round-off errors)

Max 462 lb at 15o


Lecture 20

Example

m1 = 3.50 kgm2 = 2.64 kgm3 = 8.75 kgR1 = 2.65 m @ 100o

R2 = 5.20 m @ -60o

R3 = 1.25 m @ 30o

l1 = 4 ml2 = 9 ml3 = 11 m


For the instantaneous “freeze-frame” position shown, write down the x and y components of the position vectors:R1x = r1(cos(q1)) --> do the same for R2 and R3

R1y = r1(sin(q1)) --> do the same for R2 and R3

The shaking forces and moments can be calculated using the angular velocity (100 rpm = 10.47 rad/s)Sum the moments about point OFind the position angle and mass-radius product required in plane BSolve for x and y forces in plane AFind the position angle and mass-radius product required in plane A

Lecture 20

Example

mARA = 7.482qA = -154.4o

mBRB = 7.993qB = 176.3o

learning with purpose march 25, 2013 learning with purpose march 25, 2013 22.322 mechanical design...

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