Archive for the ‘MECHANICAL’ category

Drop Forging – Introduction

September 21, 2012

Contents

    • Introduction

What is drop forging?
Drop forging is a metal shaping process, the metal to be formed is first heated then shaped by forcing it into the contours of a die, this force can be in excess of 2000 tons. The drop forging process can be performed with the material at various temperatures;

  • Hot ForgingDuring hot forging the metals are heated to above their recrystallization temperature. The main benefit of this hot forging is that work hardening is prevented due to the recrystallization of the metal as it begins to cool.
  • Cold ForgingCold Forging is generally performed with metal at room temperature below the the recrystallization temperature. Cold forging typically work hardens the metal.

There are two types of drop forging, open die and closed die.
Open die drop forging requires the operator to position the work piece while it is impacted by the ram. The die attached to the ram is usually flat or of a simple contour, most of the shaping is achieved by the operator physically positioning the work piece before each stroke of the ram. There are also special dies which can be used to cut the metal, form holes or notches. see more

Closed die drop forging comprises of a die on the anvil which resembles a mould, the ram which falls and strikes the top of the metal billet can also be equipped with a die. The heated metal billet is placed on the lower die while the ram drives down forcing the metal to fill the contours of the die blocks. see more

 

Drop Forging Diagram
Diagram of basic drop forging set up

Investment casting

September 21, 2012

Investment casting is one of the oldest known metal forming techniques, dating back over 5’000 years ago when beeswax was used to create the pattern. Beeswax back then didn’t allow the accuracy and intricate shapes we can produce today. Investment castings are used in a huge array of items, golf clubs are investment cast, aeroplanes use investment cast parts as do cars and other motor vehicles.

Creating The Mould

To begin the process wax patterns are made by injecting hot molten wax into an aluminium die, this sets the wax pattern to the exact size and shape of the required part. Many of these wax moulds are attached to a wax sprue which forms a stem linking all of the individual moulds together.
Sprue

Once the sprues are filled with the mould attachments they are dipped into a cleaning bath to ensure the future layers of shell cling to the mould profile correctly. Before the first layer of the shell is added the sprue assembly is dipped into a bath of slurry, this will form the bonding agent to the layer of ceramic powder which is added in either a rainfall sander or a fluidised sand bed, this process gradually builds up a ceramic shell around the moulds and sprue, with progressively coarser layers of ceramic coating being added over several coats until the desired shell thickness is achieved.

After the ceramic thickness is achieved the sprue along with its moulds are oven baked to both harden the ceramic shell and to melt all of the wax from within it, this will include all of the wax moulds and the wax sprue leaving a hollow ceramic shell. The wax moulds once melted leave a perfect void of the desired mould to be cast, the sprue once melted leaves open channels for the liquid metal to flow and fill all of the hollow shell (see image below). The molten metals are poured into the now hollow ceramic shell via the filling cup and left to cool off. Once the metal has cooled sufficiently the ceramic shell is broken up by either being submitted to vibrations or a water jet.

Investment Casting Shell

Advantages

Investment casting offers a number of advantages, the first being the excellent surface finish achievable, although of course the mould itself must first have these finishes. Investment casting leaves no parting or flash lines and the high quality finishes mean that the expense of another machine process to clean up surface finish is rarely required.

High dimensional accuracy is achievable which again can save a big cost on later machining to bring parts within the required tolerances.

Almost any metal can be investment cast and there is very little material waste from the process, any defect parts can usually be melted down and re used.

Investment casting has the potential to create extremely intricate parts; the molten metal will run and fill the mould very neatly, and because the castings mould is broken up to be retrieved there is little constraint to the shape of a casting.

Disadvantages

The creation of a mould can be fairly expensive and time consuming, a lot of labour is also required throughout the rest of the casting process. Occasional defects do occur for example air pockets within the casting.

Bearing Numbers Explained

September 21, 2012

  • (1) Prefix :
  • (2) Basic Number:
  • (3) Suffix

Prefix

K Cage with roller elements
L Removable bearing ring
R Ring with roller set
S Roll body of stainless steel
W Stainless steel deep groove ball bearing

*Note: Each bearing company may create their own prefixes. e.g. E2. = SKF Energy Efficient bearings

Suffix

2 RS Bearing with rubber seal on both sides. RS provides a better seal but more rolling friction than 2Z.
RS Bearing with rubber seal on one side, one side open.
2 Z / ZZ Bearing with a metal seal on both sides.
Z Bearing with a metal seal on one side, one side open.
E Reinforced Design
P2 Highest precision
K Bearing with taper bore

Bearing Numbers

The example at the header shows a 6001 2RS bearing. So what does the 6001 actually mean?
Lets attempt to break it down.

(6)001

This second number relates the bearing series, which reflects the robustness of the bearing. As you go up the scale below from 9 to 4 the inner and outer race thickness will usually increase along with the ball size, this will be to help cope with extra load.

9 Very thin section
0 Extra light
1 Extra light thrust
2 Light
3 Medium
4 Heavy

60(01)

The 3rd and 4th digits of the bearing number relate to the bore size of the bearing, numbers 00 to 03 have a designated bore size depending on the number.

00 10mm
01 12mm
02 15mm
03 17mm

While numbers over 03 simply have a bore size which is 5 times that of the 3rd and 4th digit.

This first number relates to the bearing type, as shown in the table below type 6 is a deep grooved roller bearing.

1 Self-Aligning Ball BearingThis kind of ball bearing has a spherical outer race, allowing the axis of the bearing to “wander around”. This is important because misalignment is one of the big causes of bearing failure. Self Aligning Ball Bearing
2 Barrel and Spherical Roller Bearings
3 Tapered Roller BearingDesigned to take large axial loads as well as radial loads. double row angular contact ball bearing
4 Deep Groove Double-Row Ball BearingDesigned for heavy radial loads. Double-Row Ball Bearing
5 Axial Deep Groove Ball BearingIntended for exclusively axial loads. Thrust Ball Bearing
6 Deep Groove Ball Bearing(Single row)Typical ball bearing. Handles light axial loads as well as radial loads. Single-Row Deep Groove Ball Bearing
7 Single-Row Angular Contact BearingSpecific geometry of angular contact bearing raceways and shoulders creates ball contact angles that support higher axial loads Angular contact ball bearing
8 Axial Cylindrical Roller BearingsAxial cylindrical roller bearings comprise axial cylindrical roller and cage assemblies and shaft and housing locating washers.
The bearings have particularly small axial section height, have high load carrying capacity and high rigidity and can support axial forces in one direction.

 

Axial Cylindrical Roller Bearings

Understanding Gears

September 21, 2012

What are gears used for?

Gears play a huge role in much of the technology around today. For example, car engines, drills, lathes, mills, cd/dvd players, printers, mechanical watches, children’s toys, in fact gears are almost everywhere there are motors and engines which produce a rotational movement.

Gears are excellent at keeping the rotation of two axis in sync. Where a belt and pulley system would eventually run out of sync due to a slight inaccuracy in the diameter of pulleys, a geared system will always stay in sync regardless of slight inaccuracies, this is due to the meshing of the gear teeth forcing gears to rotate consistently with one another.

•They are used to reverse direction of rotation, for example when selecting reverse in your car.
•Gears are also used to transfer rotational motion to a different axis.
•Gears are used to alter the speed of rotation which in the same way can be used to alter the end turning force or torque available.

Understanding Gear Ratios

Gear Ratio

The animation above shows two gears in mesh, imagine gear ‘A’ is driving gear ‘B’.

Because gear “A” has 20 teeth and “B” has 40 teeth “A” will travel through two complete turns for every one complete turn of gear “B” this would give a ratio of 1:2. This is because gear “A’s” rotational speed is half that of gear “B”.
If things were the other way around and gear “B” was driving gear “A” then the ratio would become 2:1 as the end rotational speed has been doubled at “A”.

Gear Types

Spur
Spur Gear
Spur gears are a simple gear type, they take the form of a cylinder or disk with their teeth formed around the gears circumference, spur gears can be meshed together on parallel axles.
Bevel
Bevel Gear
Bevel gears have a cone shape which enables them to mesh at various angles except 0 and 180 degrees, that is not to say a single bevel gear can work at multiple angles, the bevel gears must be cut to suit a specific meshing angle. The teeth of a bevel gear can be straight cut, similar to that of a spur gears teeth, or they can be curved along their length with each tooth sitting at an angle (Spiral bevel gear). Zerol bevel gears are too curved along their length but are not angled. Bevel gears are suited best to low speed applications usually sub 5m/s.
Worm drive

Worm Gear

The worm resembles the thread of a screw, and are usually meshed with a worm wheel which looks similar to a typical spur gear. Worm gears are an excellent way to increase torque output while reducing rotational speed. Worm drives have ratios varying from around 10:1 to 500:1, worm gears do have a slight disadvantage in that they are not very efficient, a lot of energy can be wasted due to the sliding action of the gear teeth. The worm itself can have 1 or more teeth, although 1 tooth that follows around the length of the worm several times can look like more than one tooth being present. A worm with one tooth is called a single thread or single start, while a worm with more than one tooth is called a multiple thread or multiple start.
Crown
Crown Gear
Crown gears are a form of bevel gears, the teeth of crown gears project at right angles to the plane of the wheel. Crown gears are usually meshed with another bevel gear, but in some instances are meshed with spur gears.
Helical
Helical Gear

Helical gears have angled teeth which form a curve that resembles a segment of a helix. Helical gears are meshed in parallel or crossed orientations and are used because they offer a more refined operation and run much smoother and quieter than spur gears for example. Helical gears can operate at high speeds and transmit large amounts of torque.

A disadvantage of helical gears is the thrust generated by the curved teeth when under load, this is usually handled by a suitable thrust bearing to help take this load.

Double helical
Double Helical
A double helical gear is similar to 2 separate helical gears joined together but mirrored, this helps eliminate the thrust that a single helical gear would create as in effect there is equal thrust in each direction cancelling each other out.