Show whole topic
May 21, 2015 11:35 am
May 21, 2015 11:35 am
Subject: Re: Brake overhaul - levers
Hi David
To add to previous comment by the other David: Lever three is the compensator on the front axle, if the holes for the brake rod clevis pins are not set at the same distance this will result in unequal pull on the brake rods and the brakes will tend to pull to one side.
I see you sent them to LMB ...Hopefully they will sort them out for you satisfactorily but should you or others need to get some made I offer the following:
Agree with Colin in so far as I believe these Girling parts would have been forgings and not castings. When the hot metal flows in the forging die and follows the shape of the component it makes it stronger due to the improved grain structure ( just like the head of a drop forged spanner).
Having said that, as a Design Engineer in a past life, I have successfully designed cast housings which were subject to very high shock loads and survived rigorous shock testing before going into service. These were made of special cast ductile iron with a minimum elongation of 12% as opposed to ordinary cast iron which has elongation typically less than 0.5%. Ductile iron is also referred to as "spheriodal graphite" cast iron. The free carbon within the grain structure is in the form of spheres whereas in ordinary cast iron the free carbon is in the form of flakes making it very brittle and a crack will readily propagate from the end of a flake which is in effect a flaw with a very small radius leading to extremely high stress at that point. NB: "Elongation" is a measure of the toughness or resistance to fracture and is the amount a test specimen stretched prior to failure when tested under tensile load.
In practice you can take a piece of 12% ductile iron, clamp it in the vice, hit it with a 4lb hammer and bend it over 90 degrees... it will behave like a piece of wrought steel. Try that with ordinary cast iron and it will snap off at the first blow!
I would be good to know the original specification that Girling used but how to discover that eh?
Whatever material is used it must be sufficiently ductile not to be prone to fracture and 12% elongation was what we accepted as "not brittle".
So tensile strength is not the only property to be considered...EN24T as mentioned by others, has elongation of 13% as well as being relatively high tensile strength so looks like a reasonable selection to me but recommend that seeking advice from a qualified metallurgist would be prudent as care needs to be taken with the heat treatment. Information I found on the internet below:
817M40T - EN24T steel is a high tensile alloy steel renown for its wear resistance properties and also where high strength properties are required. EN24T is used in components subject to high stress and with a large cross section. This can include aircraft, automotive and general engineering applications for example propeller or gear shafts, connecting rods, aircraft landing gear components.
817M40T / EN24T Steel Mechanical Properties
Size 63 to 150 mm, Tensile Strength 850-1000 N/mm², Yield Stress 680 Min N/mm², Elongation 13%, Impact 54 Izod J, Impact 50 KCV J, Hardness 248/302 HB
Hardening EN24: Heat uniformly to 823/850°C until heated through. Quench in oil.
Tempering: Heat uniformly and thoroughly at the selected tempering temperature, up to 660°C and hold at heat for two hours per inch of total thickness.
Tempering between 250-375°C in not recommended as this can seriously reduce the steels impact value.
Stress Relieving: Heat slowly to 650-670°C, soak well. Cool the EN24 tool in a furnace or in air.
EN24T steel is available from stock in round and flat bar and in plate.
One thing is for sure, there is a need to be very careful when making replacement brake parts or repairing them as if one of these failed in service and was the cause of an accident the potential consequences are horrifying!
Hope this helps
Bill
Hi David
To add to previous comment by the other David: Lever three is the compensator on the front axle, if the holes for the brake rod clevis pins are not set at the same distance this will result in unequal pull on the brake rods and the brakes will tend to pull to one side.
I see you sent them to LMB ...Hopefully they will sort them out for you satisfactorily but should you or others need to get some made I offer the following:
Agree with Colin in so far as I believe these Girling parts would have been forgings and not castings. When the hot metal flows in the forging die and follows the shape of the component it makes it stronger due to the improved grain structure ( just like the head of a drop forged spanner).
Having said that, as a Design Engineer in a past life, I have successfully designed cast housings which were subject to very high shock loads and survived rigorous shock testing before going into service. These were made of special cast ductile iron with a minimum elongation of 12% as opposed to ordinary cast iron which has elongation typically less than 0.5%. Ductile iron is also referred to as "spheriodal graphite" cast iron. The free carbon within the grain structure is in the form of spheres whereas in ordinary cast iron the free carbon is in the form of flakes making it very brittle and a crack will readily propagate from the end of a flake which is in effect a flaw with a very small radius leading to extremely high stress at that point. NB: "Elongation" is a measure of the toughness or resistance to fracture and is the amount a test specimen stretched prior to failure when tested under tensile load.
In practice you can take a piece of 12% ductile iron, clamp it in the vice, hit it with a 4lb hammer and bend it over 90 degrees... it will behave like a piece of wrought steel. Try that with ordinary cast iron and it will snap off at the first blow!
I would be good to know the original specification that Girling used but how to discover that eh?
Whatever material is used it must be sufficiently ductile not to be prone to fracture and 12% elongation was what we accepted as "not brittle".
So tensile strength is not the only property to be considered...EN24T as mentioned by others, has elongation of 13% as well as being relatively high tensile strength so looks like a reasonable selection to me but recommend that seeking advice from a qualified metallurgist would be prudent as care needs to be taken with the heat treatment. Information I found on the internet below:
817M40T - EN24T steel is a high tensile alloy steel renown for its wear resistance properties and also where high strength properties are required. EN24T is used in components subject to high stress and with a large cross section. This can include aircraft, automotive and general engineering applications for example propeller or gear shafts, connecting rods, aircraft landing gear components.
817M40T / EN24T Steel Mechanical Properties
Size 63 to 150 mm, Tensile Strength 850-1000 N/mm², Yield Stress 680 Min N/mm², Elongation 13%, Impact 54 Izod J, Impact 50 KCV J, Hardness 248/302 HB
Hardening EN24: Heat uniformly to 823/850°C until heated through. Quench in oil.
Tempering: Heat uniformly and thoroughly at the selected tempering temperature, up to 660°C and hold at heat for two hours per inch of total thickness.
Tempering between 250-375°C in not recommended as this can seriously reduce the steels impact value.
Stress Relieving: Heat slowly to 650-670°C, soak well. Cool the EN24 tool in a furnace or in air.
EN24T steel is available from stock in round and flat bar and in plate.
One thing is for sure, there is a need to be very careful when making replacement brake parts or repairing them as if one of these failed in service and was the cause of an accident the potential consequences are horrifying!
Hope this helps
Bill