Beau Wrobel Kyle Mortensen
Lawnmower Blade Case Study Description We will be looking into a lawnmower blade for a higher end OM use for brands such as a s !ohn Deere" #us$%arna" etc& 'his lawnmower blade will also ser%e as a mulcher which may change which type of material to use in the end&
Operational Conditions 'his metal alloy will be sub(ected to %arious types of conditions conditions dependent upon where in the world it will be used& We had thought of these few things that the blade would be sub(ected to fairly $uickly and thought we were lacking in the conditions it would be sub(ected sub(ect ed to& 'emperatures can range from )* degrees + to ,-* degrees +& 'his blade will also be sub(ected to chemicals and pesticides found in the grass as well as moisture and chemicals in the outside air& air& 'he blade will also ha%e to withstand sand" mud" and other things that may be in the soil& 'he blade itself will be sub(ected sub(ected to di.erent types types of loads such as impact" torsional" tensile and compressi%e/tensional compressi%e/tensional loads& We think that the impact load will withstand the highest load when the blade comes into contact with unwanted materials such as rocks" roots" etc& We feel that this is the most important property as a higher toughness in the metal selected will add to the ser%ice life of the blade which will gi%e our blade a better reputation& 'he blade will also ha%e to withstand rpms up to )*** rpm which will e.ect tensile and torsional loads& 'he blade will also ha%e to dampen %ibrations as well until the blade can no longer be used& 0nother impact load it will e1perience is a grinding wheel when being ser%iced to sharpen the blade&
Beau Wrobel Kyle Mortensen
+ailure Modes +or our blade during standard operation" this blade could e1perience plastic deformation from warping or bending from hitting ob(ects or from (amming up the mower deck& 'he blade can also fracture from hitting harder ob(ects such as rocks" or from corrosion& 2ocks can also apply impact loads that will not fracture the blade" but instead cause dents and Knicks which may damage the blade beyond repair&
Manufacturing Methods One manufacturing method and the main method to produce our lawnmower blade is to blank" form and mill our blade& 'his process in%ol%es raw coiled steel that is blanked to si3e which is a pressing method to press out a speci4c amount of material& 'he shape of the blade will then be milled to our speci4c dimensions then be C5C milled for 4ner parts such as the blade& 'he blade will then go through a special heat treating process called 0ustempering which will be e1plained in detail below& 'he blade then goes through an inspection for any 6aws that may occur during the pre%ious process& 'he blade will then 4nally go through a coating and labeling process in which it can be coated with a wide %ariety of coatings to pro%ide corrosion resistance and surface protection from an outside pro%ider and it will be labeled for part numbers" logos and bar codes& 0ustempering is a heat treating process for medium7to7high carbon ferrous metals which produces a metallurgical structure called bainite& 8t is used to increase strength" toughness" and reduce distortion& 9arts are heated to the hardening temperature" then cooled rapidly enough to a temperature abo%e the martensite start temperature and held for enough time to produce the desired bainite microstructure according to Bodycote&com& 'his process is particularly useful for thinner metal materials such as lawnmower blades as it reduces distortion" increases toughness" strength" and increases resistance to shock&
Beau Wrobel Kyle Mortensen
Material 9roperties 2e$uired Our lawnmower blade will re$uire se%eral di.erent properties to meet the demands of higher end OM manufactures& 'hese properties include: • • • • • • •
#igh #ardness Lower Ductility #igh Corrosion 2esistance #igher 'ensile Strength #igh 8mpact/+racture 'oughness #igh Wear 2esistance #igh Modulus of lasticity We are looking for a hardness %alue around -* #2C" tensile strength %alues near ,**"*** psi ;'S" and se%eral other numbers that will be determined for the 4nal decision matri1& We 4gured that we would need a higher Modulus of lasticity so that the blade would not easily bend when sub(ected to harder ob(ects that may be found while mowing&
Desired Many of our material properties are listed abo%e in the re$uired but some properties that would be nice to ha%e are: • • •
Decision Matri1 +or our decision matri1" we found it di=cult to 4nd tabulated data for our three metals that we selected to choose from& 'hese metals that we decided to choose from were ,*>* steel" -,?* steel and O, tool steel& We
Beau Wrobel Kyle Mortensen
choose these three metals from research on what types of metals were generally used for lawnmower blades and cutting applications& 'his matri1 was di=cult to construct as much of the decision making was based on generali3ed statistics such as the property being fair" good" medium" etc& 'he data displayed below is what we could 4nd on the steels that we choose and the importance we felt that each property was to the application&
Propertie s
Importa 1080
5160
O1
nce
****
****
****
lastic Modulus @KS8A ield Strength
F)***
@9S8A
>)>** ,
EE** ,
* ?*
E
#ardness
#2C Mediu
#2C
#2C Mediu
>
'oughness Machineabil
m
+air
m
,*
ity
9oor
-
Weldability Corrosion
9oor
m
)
2esistance Wear
Low
Low
Low Mediu
?
2esistance
+air
+air
m
E
'he math was not necessary for our matri1 as it was clear that the O, tool steel was our clear winner& 8ts closest ri%al was the ,*>* steel" which if
Beau Wrobel Kyle Mortensen
we were to e1pand our business of making lawnmower blades into cheaper markets" this would be our alloy of choice& #owe%er" the blade we are looking to make is highest of $uality and the O, has the better material properties for the (ob at hand& Before going into the decision matri1 we had a sense that some sort of tool steel would be our choice in alloy based on prior knowledge of the di.erences between regular steel and tool steel and with some of our research we found in 4guring out what material properties were needed as well as its operating en%ironment& We disco%ered that O, tool steel is used in cutting applications like knifes" much like the ,*>* steel" but with the added bene4ts of tool steel such as higher hardness" toughness and better machine7ability from what we researched&
9rototype 'esting +or prototype de%elopment of our lawn mower blade we chose to only make our prototypes out of the 0ustetempered O, tool steel as it was by far the best material for our application& 8n order to 4nd the absolute limit we can push our lawn mower blade to we decided to sub(ect it to a series of tests& One being a test in which the blade would be spinning at operating 29M and it would be sub(ected to an impact by a granite stone much like one that would be found in a common back yard application& 'his test is being performed to test the impact toughness of the alloy along with hardness" wear resistance and sti.ness& 'he alloy will be tested by ha%ing a stone dropped from a constant height and set location into the spinning blade& 'he height will be constant to simulate a stationary rock being impacted" howe%er the location of the impact will be changed at a constant rate to test the rigidity of the alloy at a certain point& 'his test will be performed until fracture occurs" the si3e of the stone will be increased with each consecuti%e test& 'he test will also be performed at temperatures %arying from *7F-*G+& 0fter each test is performed before and after fracture the sharpness of the blade will be tested to ensure the edge does not dull at an accelerated rate&
Beau Wrobel Kyle Mortensen
0nother test that will be performed on the mower blade will be sub(ecting the blade to small rocks" sand and gra%el in order to test the hardness and wear resistance of the blade& 'his will be done by spinning the blade at operating 29M and then pouring a set amount of rocks" gra%el and sand into it" the blades 29M will be held constant for a set amount of time with the gra%el and sand in it& 0fter the test time is o%er the blade will be tested for sharpness and for wear resistance by looking for chips in the powder coat surface of the blade" the sharpness test would be performed by sending the blade to an independent test company before during and after each test" to pro%ide $uantitati%e date for the sharpness of the blade& ach consecuti%e test will ha%e an increased time with the sand" gra%el" and stones in the blade& 'his test will continue until an e1cessi%e amount of wear has been recorded&
Bibliography AZOM& F 0ugust F*,F& ,F 0pril F*,-& Bodycote& HBodycote&H n&d& bodycote.com. ,- 0pril F*,-& e+unda& eFunda& n&d& ,F 0pril F*,-& EngineersEdge & , !anuary F***& ,F 0pril F*,-& +isher Barton& H+isher Barton South Carolina &H , !anurary F*,-& fsherbartonsc.com. , 0pril F*,- &
