MANE-4030 Elements of Mechanical Design Sample Final Exam Questions Prof. A. Maniatty

MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
1. A belt-driven jack-shaft is shown schematically in the figure. The weight of each pulley is
900 N as shown in the figure. The shaft is made of AISI 4620 cold-drawn (CD) steel (a
hardened steel) and is driven by a motor at 1200 rpm. The pulleys are held in place by sledrunner keys. Assume the bearings at A and C only support normal loads, no moments. All
important surfaces have a ground finish. If the shaft is to be designed for an infinite life with a
reliability of 99.9% and a safety factor of 1.5, what is the required diameter at location B. (26
points)
keyway
B
C
D
A
450 mm
450 mm
225 mm
keyway
MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
2. A typical bolted joint of the type shown uses a MC 12 x 1.75 Class 5.8 steel bolt with a fatigue
strength of Sf = 340 MPa (assume this strength has not been modified for stress concentration
effects). The length of the bolt is selected so that the threads stop immediately above the nut.
The bolt clamps two 25-mm thick steel flanges together with a 1.5-mm lead-alloy gasket
(Young’s modulus for the lead is E = 37 GPa) between the flanges. A bolt preload of 20kN is
used. The external separating load fluctuates cyclically from 0 to 12kN. The thread fatigue
stress concentration factor is Kf = 3. Assume the effective load carrying area of the steel flanges
and of the lead gasket may be taken as 450 mm2.
(a) Find (i) the part of the externally applied load that
is carried by the bolt (i.e. what fraction of the load that
cycles from 0 to 12 kN is carried by the bolt) and (ii)
the maximum and minimum overall bolt forces
(including the preload). (10 points)
(b) Find the safety factor guarding against fatigue failure if designing for an infinite life. (8
points)
MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
3. The figure shows a planetary gear train, which is driven by a 40 kW multicylinder engine at a
constant speed of 1200 rpm in the direction shown and drives a concrete mixer (moderate shock).
Tooth numbers for the gears in the figure are N2 = 21, N3 = 35, N4 = 21. The gears have a module
of 5 mm and a 20° pressure angle.
ωin
€
(a) How many teeth must gear 5 have, and what is the speed and direction of output shaft b
(show direction on the diagram above)?(7 points)
(b) Draw a free body diagram of ring gear 5 showing the forces and torque acting on the gear.
Compute values for the forces and torque. Assume 98% efficiency and that there are just 2 of
each planet gear. (10 points)
MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
4. A 360° hydrodynamic journal bearing is to support a shaft rotating at 3600 rpm with a radial
load of 3 kN. The bearing sleeve has a 32-mm diameter and is 32-mm long. The lubricant
chosen is SAE 10 oil, and the average operating temperature at the bearing interface is estimated
to be 60°C (140°F). Minimizing friction is of primary importance in this application.
(a) Determine the clearance that should be specified in this application. (8 points)
(b) Estimate the power loss from bearing friction. (8 points)
MANE-4030 Elements of Mechanical Design
Sample Final Exam Questions
Prof. A. Maniatty
5. Short answer.
(a) When designing a rotating shaft, in addition to yield and fatigue strength considerations, the
system dynamics should also be considered. What problem can arise due to the rotational
dynamics, and what may be done to prevent this problem? (4 points)
(b) A 1.5-inch diameter shaft is to be supported with a nylon bearing sleeve. The maximum
radial load to be supported at the bearing is 300 lb. The shaft rotates at 50 rpm. A bearing length
of 1.5 inches is proposed. Is the nylon bearing sleeve acceptable for this application. (6 points)
(c) Name one advantage of using a weld instead of a threaded fastener to form a joint? (2 points)
(d) Name one advantage of a rolling element bearing as compared to a sliding contact bearing?
(2 points)
(e) A free body diagram of a shaft is shown in the figure. Compute the stresses, σ xx , σ zz , τ xz , at
point A for a material element on the top of the shaft parallel to the x-z plane. The shaft diameter
at A is 25 mm. Dimensions shown are in mm. (9 points)
€
0.0833 kN
1.25 kN
T = 0.2 kN-m (acting about –z axis)
2.25 kN
T
0.5833 kN