Great Mechanical Engineering Tips

These mechanical engineering tips are generally available in textbooks …… this is just to brush up ….. Whilst working in industry for last 20 years, I found youngsters have forgotten many basic things…. We all are required to read books on fundamental principals…..We don’t know when we are going to require the knowledge we learned in past ….. Let’s be prepared in the edge of Robotics…Working Experience can not be excuse for brushing your knowledge.

Machinery Foundation

Definition of foundation: A foundation generally may be defined as the base or support upon which anything may rest. A building for example is supported by a base (concrete or other strong material) at its lowest point; it is this base which is termed as foundation.

There are certain fundamental requirements which are common to all machinery foundations, and these are follows:

  • They must carry the applied load without any settlement or crushing.
  • They must maintain true alignment with any power communication mechanism.
  • They must absorb all the vibrations and noise which may be caused by inertia or unbalanced force.
Safe Load on various substances
Substance
Tons / ft²
Made up of ground
0.5
Quick sand, alluvial soil etc
0.5 to 1
Soft clay
1 to 2
Ordinary earth
1.5 to 2
Clay in thick bed or dry
2 to 4
Soft friable rock
3 to 5
Hard coarse gravel
4 to 7
Clay in thick beds, dry
4 to 6
Ordinary rock
5 to 15
Firm coarse sand and gravel
6 to 8
Hard compact rock
20 to 30

 

Safe Load on foundations material
Material
Tons/ft²
Portland Cement, Concrete 5:1
15
Portland Cement, Concrete 10:1
7.5
Concrete, 1:3:6 @ 60 days
6
Concrete, 1:2:4 @ 60 days
10
Ordinary Mortar
3.5
Bricks in ordinary mortar
3.5
Bricks in cement mortar
3.5
Blue bricks in cement mortar
5.75
Portland cement and sand mortar 1:3
15
Granite
72

Excavation necessary should be in every case be continued until a fine and fine and solid sub soil has been found or this be impossible other methods such as piling or other strengthening may have to be restored to. If you are confused contact qualified Civil Engineer and avoids guess work.

Foundations for heavy machinery are generally constructed of Solid Concrete, Reinforced Concrete, and Structural Steel.

Gas and Oil Engine Foundation

How to calculate Load on foundation?

1

Load on foundation = Dead Load + Vacuum Load (for condensing units) + Wt. of foundation members + impact load

Foundation Bolt or anchor bolts are squiring the machine rigidity to its foundation. Normally the foundation sizes are given in installation manual of machine to be installed.

Power, Torque and Work

Work = Force X Distance
Power = Work / Time
Horse Power (HP) = FNR2Π / 33000 = FNR / 5252 where F = Force, N = rpm, R = radius Torque = F x R, HP = TN / 5252
Refer Machinery Hand Book for more detailed explanations.

2

Belts and Pulleys:

Belt is defined as continues strip or band utilize for transmission of power from one pulley to another pulley. Generally belts are made up of Lather, Rubber, Cotton, Manila , Hemp, Steel.

The pulley transmit power is called as driver pulley and which receive the power is called as driven pulley. The tendency of slipping, especially under heavy loads is often beneficial in that it will absorb a portion of shock of suddenly applied loads and thus protect, to some extend both driving and driven machine. Tanned or water proofed lather belts, fabric or rubber belts may be selected to meet particularly any atmospheric condition. In the installation of lather belts, precaution should be taken to put flesh side on the outside and the grain and smooth side towards pulley.

V belt is endless belt, running on V formed grooves formed into the pulley surface.

The belt speed in feet/min = Π x pulley diameter in inch x rpm / 12

Length of open belts = (Π D/2) + (Π d /2) + 2√{C² + [(D-d)/2]²}

Where D = diameter of large pulley, d = diameter of small pulley, C = center distance between pulley.

3

For Cross belts the Length = [Π + (Πxφ/90)] [ (D+d) / 2] + 2 C cos φ
φ = in radians

Strength of Belts

4
HP = (T2-T1) x V / 33000

Where,
V = Π x D x N
D = pulley diameter
N = revolution per min
Π = 3.141572

Belt Width: Bw = HP x 800 / V

Pulley Diameter Vs Belt Thickness

Type of Belt
Thickness (inch)
Min Diameter of Pulley (inch)
Light Single 1/8 to 5/32 3
Medium Single 5/32 to 3/16 4
Heavy Single 3/16 to 7/32 5
Light Double ¼ to 9/32 6
Medium Double 5/16 10
Heavy Double 3/8 16
Medium Three ply ½ 30
Heavy Three ply 9/16 36

Millwright’s Rule:

  1. A single belt 1 inch wide running @ 800 ft/min will deliver approximately 1 HP
  2. A double belt 1 inch wide running @ 500 ft/min will deliver approximately 1 HP

Belt Joints
Method of making joints:

  1. Cementing
  2. Lacing which may sub divided into various classes such as lap, butt, apron joints
  3. Patented hook and plates

Drive arrangement and Belt Maintenance
Horizontal drives are always preferred. The sag of the upper side tends to increase the arc of contact of pulley. This sag should be 1 ½ inch for every 10 ft. of the center distance between the shafts. Vertical drive should be avoided when the lower pulley is smaller. The best arrangement is 45º.

Necessary Information for estimate and recommendation of Belt Drive:

  • H.P.
  • Size of both pulley
  • Speed in RPM
  • Shaft Center distance
  • Belt open or close
  • Type and position of idler if used
  • Inclination of drive (Angle in Deg.)
  • Slack Side of belt (Top or Bottom)
  • Type of pulley (stepped or flanged)
  • Nature of drive, type of machinery
  • Load fluctuation etc.

Belt Maintenance: Keep the belts tight, taking up of slack belts, running in new belts, cleaning of dirty belts, dressing of belts, maintenance of belt guards

Shafts, coupling and bearing

Rotating members may, however, be classified roughly according to the particular purpose for which they are intended as;

  • Axles loaded transversely and subjected principally to bending.
  • Shafts subjected to torsion or combination of torsion and bending.
  • Spindles which are directly carry a tool for actually doing work and as such must have accurate motion.

Shaft subjected to torsional stress only.
Factor of safety: For head shaft = 15, line shaft = 10, countershaft = 7

Allowable stress in steel shaft for above factors is approx. 4000, 6000, and 8500 respectively.

Shaft Dia Type of Shaft
d = 4.3 (HP/N) ⅓ Head Shaft
d = 3.75 (HP/N) ⅓ Pulley carrying line shaft.
d = 3.36 (HP/N) ⅓ Short outer shafts

Shaft subjected to bending only.

Y = P x L³ / (3 x E x I) For cantilever beam
Y = P x L³ / (48 x E x I) For concentrated load at middle
Y = 5 W x L³ / (384 x E x I) For uniformly distributed load

Where,
Y = maximum deflection of beam in inches
P = load in pounds
W = total uniform load in pounds
L = length of beam
E = modulus of elasticity pounds/inch²

Shaft Coupling
Shaft coupling are used to fasten together the ends of two shafts so that the motion from one shaft may be transmitted to other.

Coupling may be classified according to their function as permanent coupling, release coupling.

Flange coupling: n = 3+ (D/2)

Where,
n = no of bolts
D = diameter of shaft in inch.

Universal coupling: Used when power transmitted is inclined or declined to the plane of one shaft. Main application is in automobile.

Bearing:
Bearing may be divided into two general classes as Journal Bearing. Thrust Bearing.
Refer Manufacturers Catalog for installation and maintenance of the Bearing.

Calculation tips for Mechanical Engineers

Electrical Formulae

 

Required DC
AC (Single Phase)
AC (Two-Phase [4 wire])
AC Three Phase
Amp when HP is known (HP x 746) / (E x EFF) (HP x 746) / (E x EFF x PF) (HP x 746) / (2 x E x EFF x PF) (HP x 746) / (1.73 x E x EFF x PF)
Amp when Kilowatt known KW x 1000 / E KW x 1000 / (E x PF) KW x 1000/ (E x PF x 2) KW x 1000 / (E x PF x 1.73)
Amp when KVA is known KVA x 1000 / E KVA x 1000 / (2 x E) KVA x 1000 / (1.73 x E)
Kilowatt I x E / 1000 I x E x PF / 1000 I x E x PF x 2 / 1000 I x E x PF x 1.73 / 1000
KVA I x E / 1000 I x E x 2 / 1000 I x E x 1.73/ 1000
HP output I x E x EFF / 746 I x E x EFF x PF / 746 I x E x EFF x PF x 2 / 746 I x E x EFF x PF x 1.73 / 746

Where,
I = Amp
E = Volt
EFF = Efficiency expressed as decimal
HP = Horsepower
PF = Power factor
KW = Kilowatt
KVA = Kilovolt –amp

Brake HP

Brake HP = PV / 33000

Where,
P = Weight in pounds
L = length of lever arm in feet from the center of shaft
N = rpm
V = 2 x Π x L x N = velocity at point in feet/min @ distance L

Water HP

Water HP = Q x W x H / 550

Where,
Q = water flow [ft³/ sec]
W = wt of cubic unit of water [lbs / ft³]

Engine HP

Engine HP = PLAN x K / 33000

Where,
P = effective pressure acting on piston
L = length of stroke
A = Area of piston in inch²
N = No of working stroke / min
K = coefficient equal to ½ times number of cylinders in gas engines and in double acting steam engines 2 times the number of revolution.

Boiler HP
The equivalent evaporation of 34.5 lbs of water /hr from feed water temperature of 212ºF into dry stem at the same temp is defined as boiler HP. However, since the actual operating conditions of boiler are seldom from and at 212ºF a factor is needed to convert the rating from the actual conditions to what be from and at 212ºF. This is called factor of evaporation and equals to the ratio of heat required to generate one pound of steam under actual conditions to that required to make one pound of steam from and at 212ºF

Cº = (5/9) (Fº -32)

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