The goal of material selection is to c ome up with an appropriate material that best meet s the design requirements. The approach is to identify the connection between functional requirements and the material properties so as to help us reduce the number of candidate materials from which to select from.
The following are components and materials required in the design of a power screw (screw jack):
3.3.1 Frame (Body)
Most of the frames are in conical shape and hollow internally to accommodate both the nut and scre w assembly. The frame works to ensure that the screw jack is safe and has a complete rest on the ground.
The purpose of the frame is to support the screw jack and enable it to withstand compressive load exerted on it.
The frame is a bit complex and thus requires casting as a manufacturing process. For this reason, grey cast iron as a material is selected for the frame. This is also evident from the case study on previous design of the same product (Nyangasi, 18 Decem ber, 2006). Cast iron is cheap and it can give any
complex shape without involving costly machining operations. Cast iron has higher compressive strength compared to steel. Therefore, it is technically and economically advantageous to use cast iron for the frame. Graphite flakes cast iron with an ultimate t ensile strength of 220MPa is considered suitable for the design of the frame. The graphite flakes improve the ability to resist compressive load.
Mechanical properties British Standard Specification
Tensile strength (MPa) 220
Compressive strength (MPa) 766
Shear strength (MPa) 284
Endurance limit (MPa) 96
Young’s modulus (GPa) 89 – 114 Modulus of rigidity (GPa) 36 – 45
Hardness number (HB) 196
Table3.1: Mechanical Properties of Cast iron – Appendix A (Marshek, 2012
3.3.2 Screw
The screw is subjected to tensional moment, compressive force and bending moment. The screw profile is square type because of its higher efficiency and self-locking but not compared to trapezoidal threads.
Square threads are usually turned on lathes using a single point cutting tool also square threads are weak at the root and this leads to use of free cutting steel. Screws are usually made of steel where great resistance to weather or corrosion is required. Most fasteners close to 90% use carbon steel because steel has excellent workability, offers abroad range of attainable combinations of strength properties and it is less expensive. Medium plain carbon steel can be heat treated for the purpose of improving properties such as hardness, strength (tensile and yield), the desired results are therefore obtained (Fasteners, 2005). This leads to the use of plain carbon steels.
Table3.2: Mechanical Properties of Plain carbon steel – Appendix B ( Nyanja’s , 18 December, 2006) 3.3.3 Nut
There exists a relative motion between the screw and the nut which causes friction, friction in turn causes wear of the material used for screw and nut. Therefore, it requires one of the two members to be softer. A suitable material for the nut is therefore phosphor bronze which is a copper alloy with small percentage of lead and has the following advantages;
Good corrosion resistance.
Low coefficient of friction.
High tensile strength.
Bronze has 0.2% phosphor to increase te nsile strength and the yield stresses may be taken as; tension = 125MPa, compression = 150MPa, yield stress in shear = 105MPa with safe bearing pressure of 15MPa, ultimate tensile strength is 190MPa and a coefficient of friction of 0.1.
Table3..3: Safe Bearing Pressures for Power screws – Appendix C (Nyangasi, 18 December, 2006) &
(Gupta, 2005 3.3.4 Handle
The handle is subjected to bending moments so plain carbon steel of BS 080M30 with yield strength of 385MPa can also be used. It has the same mechanical properties and process as in Table 3.2.
3.4.4 Cup
Shape of cup is complex and thus re quires casting process. It also has the same properties as in Table3.1.
Taking graphite flakes cast iron with an ultimate tensile strength of 200MPa. The graphite flakes improve the ability to resist compressive load.
3.4.5 Set Screw and Lock nut + Washer
The purpose of the set screw is to resist motion of nut with screw. The lock nut + washer on the other hand is used to provide uniform force by e nlarging the area under the action of the force. We can use plain carbon steel for both and they have the same manufacturing process and properties as in Table 3.2
CLASSIFICATION OF SCREW THREADS 3.1 Introduction
Screw jacks commonly use various forms of threads, namely; square threads, ISO metric trapezoidal threads and buttress thread.
3.1.1 Square Thread
As the name suggest, it has a square cross section of the thread. It is the most common form used by the screw jack and used especially in high load applications.
3.1.1.1 Nomenclature of Square Thread
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Figure 3.1: Nomenclature of square thread 3.1.1.2 Advantages of the Square Thread The advantages of square threads are as follows:
(i) They have high efficiency.
(ii) They have lower friction coefficient hence less power loss in lifting the load.
(iii)Motion of the nut is uniform since there is no side thrust and radial pressure on the nut.
3.1.1.3 Disadvantages of Square Thread
The disadvantages of square threads are as follows:
(i) The threads are usually turned on a lathe machine with a single point cutting tool hence e xpensive compared to machining with multi-point cutting tools. This makes them more difficult to manufacture.
(ii) The strength of a screw depends upon the thread thickness at the core diameter. Square threads have less thickness at core diameter than trapezoidal threads. This reduces the load carrying capacity of the screw.
(iii) It is not possible to compensate for wear in square threads since wear of the thread surface
becomes a serious problem in the service life of the power screw. Therefore, replacement of the nut or the screw is required when worn out.
Applications: Square threads are used for screw-jacks and presses.
3.1.2 ISO Metric Trapezoidal Threads
These are threads with trapezoidal outline profile. They are most commonly used for lead screws. They offer high strength and ease of manufacture.
3.1.2.1 Nomenclature of ISO Metric Trapezoidal Thread
Figure 3.2: Nomenclature of ISO metric trapezoidal thread 3.1.2.2 Advantages of the Trapezoidal Thread
(i) They are cheap to manufacture as compared to square threads. Multi-point cutting tools are employed for machining compared to single point cutting tools that ar e used in machining square threads.
(ii) The trapezoidal thread has greater t hickness at core diameter than that of the square thread.
Therefore, a screw with trapezoidal threads is stronger than an equivalent screw with square threads.
Such a screw has large load carrying capacity.
(iii) The axial wear on the surface of the trapezoidal threads can be compensated by means of a split-type of nut. The nut is cut into two parts along the diameter. As we ar progresses, the looseness is prevented by tightening the two halves of the nut together. The split-type nut c an be used only for trapezoidal threads. It is used in lead-screw o f lathe to compensate wear at periodic intervals by tightening the two halves.
3.1.2.3 Disadvantages of Trapezoidal Threads
The disadvantages of trapezoidal threads are as follows:
(ii) Trapezoidal threads result in side thrust or radial pressure o n the nut. The radial pressure or bursting pressure on the nut affects its performance.
Application:Trapezoidal and acme threads are used for le ad-screw and other power transmission devices in machine tools.
3.1.3 Buttress Thread
Figure 3.3: Nomenclature of buttress thread 3.1.3.1 Advantages of Buttress Thread
The advantages of buttress threads are as follows:
(i) It has higher efficiency compared to trapezoidal threads.
(ii) It can be economically manufactured on a thre ad milling machine.
(iii) The axial wear at the thread surface can be compensated by means of split-type nut.
(iv) A screw with buttress threads is stronger than equivalent screw with either square threads or trapezoidal threads. This is because of greater t hickness at the base of the thread.
3.1.3.2 Disadvantages of Buttress Thread
The buttress threads have one disadvantage. They can transmit power and motion only in one direction as compared to square and ISO metric trapezoidal threads, which can transmit force and motion in both directions.
Application: Buttress threads are used in vices, where force is applied only in one direction. 12
3.2 Thread Series
There are three standard thread series in the unified screw thread system;
Fine series Coarse series
Normal series
Fine thread series have more threads per axial distance and thus have a smaller pitch while coarse thread series have a large pitch (fewer threads per axial distance). This shows that fine series threads are stronger as compared to coarse thread series of the same dimensions (diameter) (Fasteners, 2005).
Fine series has advantages over the other series, these are;
They have large stress areas hence are strong in compression.
They have a larger minor diameter which de velops higher tensional and shear strength.
They have smaller helix angle therefore pe rmitting closer adjustment accuracy.