Babcock International Group PLC. Capstans A Matter of Friction. Dr Helen Meese Technical Assurance Engineer. trusted to deliver

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1 Babcock International Group PLC Capstans A Matter of Friction Dr Helen Meese Technical Assurance Engineer

2 Introduction Babcock International Group Babcock Marine Who am I? Why is Engineering so cool? The Problem Dispensing Cable The Science Overcoming friction Slide 2

3 Babcock International Group Babcock Marine Babcock Integrated Technology Turnover ~ 700M pa & 7,200 employees the major RN support company Sole submarine platform in-service support contractor Provides 75% of current surface ship support services provision of specialist consultancy services New equipment supply to: CVF Aircraft Carrier Programme Type 45 Frigate Programme Astute Submarine Programme Eurofighter Typhoon & Tornado Programme Slide 3

4 Surface Ships & Specialist Equipment Slide 4

5 Surface Ships & Specialist Equipment Problem Solving Capability Client Requirements Capture Concept Design to In-Service Support Engineering Resources Mechanical Electrical and C&I Software Safety and Risk Analysis Analysis (stress, seismic, thermal, physics & shielding) Support for site installation, testing and commissioning Support Tools 3D CAD modelling AutoCAD Mathematical modelling FEA EMC testing Slide 5

6 Who am I? Studying was hard work Only Got One A-level Went through Clearing Offered place on Foundation Engineering Course Got 2:1 in Electro-mechanical Power Engineering Offered a PhD in Turbomachinery Worked for Babcock since 2005 Slide 6

7 Why Engineering? Interesting Why things work Challenging solutions to complex problems Variety Types (Mechanical, Civil, Electrical, Chemical etc.) Industries (Energy, Utilities, Telecoms, Environment) Projects (small and large scale) Job Satisfaction Providing useful solutions See your work in action Great Opportunities Slide 7

8 The Problem How can we pull a heavy load?

9 Controlling Heavy Loads - Cable Dispensing Frictional forces play an important role in rope dispensing Friction force helps control the speed of the object being dispensed However, frictional force fights against the object during the dispensing process Since friction in rope dispensing can change exponentially with the rope geometry and the coefficient of friction, understanding the factors that affect rope friction is essential Slide 9

10 Capstans Slide 10

11 Cable Dispensing Applying a simple friction law derived for a capstan we gain a better understanding of the behaviour of these devices Rope dynamics generated by friction can be estimated given the amount of rope, the weight of the load, the rope modulus, and the frictional force Slide 11

12 Science Friction

13 The Basics - Friction Law Tangential forces generated between contacting surfaces are known as frictional forces These tangential forces resist motion up to a point Tangential force F f is proportional to the normal force F N for impending motion where F f F N is called the coefficient of friction Normal Force F N Tangential Force F f Once the maximum frictional force is exceeded, then sliding will occur Slide 13

14 Friction Law - Sliding The ratio of the tangential frictional force to the normal force for no slipping is called the static coefficient of friction s For sliding surfaces, the term dynamic coefficient of friction k is used Slide 14

15 Friction Law - Sliding Blocks Consider two blocks sliding across a floor Even though the blocks are orientated in different directions & have different contact areas, the amount of force required to start the block sliding will be the same Why? Because the force required to slide the blocks depends only on the weight of the block and the coefficient of friction between the block and the sliding surface To understand this, consider the normal stress defined as: N F N A Slide 15

16 Sliding Blocks Re-arranged it is F N A N If we consider the friction stress to be N and the frictional force to be F f = N A Then the ratio of Tangential to Normal friction can be written: F F f N N A A When the limiting tangential stress is assumed proportional to the normal stress, the areas cancels to give the ratio of the tangential force to the normal force independent of the contact area Friction law is independent from contact area N Slide 16

17 Time to Think How can we find out the coefficient of friction of an object? What happens when we add a non-stick surface? How can a man pull a bus? Slide 17

18 Capstans Moving Loads with Ease

19 Capstans Affecting Forces If we want to make moving an object easier we can use a capstan The Capstan equation is an Exponential Function and the differential forces for tension can be determined by considering a rope over a drum The Capstan Equation is defined as where T1 is the Load, T2 is the Back Tension, is the Contact Angle (in Radians) & is the Coefficient of Friction Handout provided T2 T1 e Slide 19

20 Load (N) Number of Turns on Capstan Capstans - Graphically The friction will increase exponentially with the coefficient of friction and the contact angle Just like the sliding contact block, the solution is independent of the contact area, and independent of the radius of bend and the size of the rope Change in Load for Varying Coefficient of Friction: Number of Turns, Back Tension & Angle held constant Coefficient of Friction T /4 3 2/4 3 2/4 3 1/ /4 2 2/4 2 2/4 2 1/ /4 1 2/4 1 2/4 1 1/4 1 No of Rope Turns with Varying Coefficient of Friction: Load and Back Tension constant Coefficient of Friction No Turns Slide 20

21 Capstans - Affecting Forces So why is this important? Friction can vary greatly depending on the ropes condition; wet, muddy, worn And on the surface the rope is in contact with; smooth, toothed Rope slip can cause the load to run-away, this could be a disaster The number of rope turns can effect the ease at which the load can be moved Slide 21

22 Time to Think In theory, using a Capstan a toddler could be capable of holding the weight of two USS Nimitz Supercarriers (97,000 tones each) and it would feel like only 1 kg!! How can this be measured? When do the number of turns on a capstan become impractical? Slide 22

23 Turning it into a Lesson

24 Mathematical Topics Covered Based on the Maths syllabus: Level 3 Certificate in Mathematics for Engineering H860, the following areas have been covered: Sec 2.1 Using Mathematical functions related to growth and decay Sec 2.2 Solving problems with Exponential growth & decay Sec 2.3 Setting up and solving differential equations to model a physical situation Sec 7.1 Solving engineering problems using vector methods Sec 8.1 Summarising a set of data Slide 24

25 Your Turn. Use real life facts Use experiments Make it fun Any Questions? Slide 25