View Full Version : Bolt grade for blade?

Vibe Ray
03-08-2001, 11:30 PM
Which grade bolt is the best for the blade bolt that holds the blade to the spindle?

03-08-2001, 11:33 PM
Did you get your ZTR yet?

03-08-2001, 11:48 PM
I think most decks use Grade 5, that's 3 marks on the head. You could just as easily use grade 8, that's 6 marks on the head. The 8's would wear down slower than the 5's and would probably cost 50% more. I like to put a little anti-seize on mine and torque them.

Eric ELM
03-08-2001, 11:53 PM
The bolts that come with it should be heavy enough. As Bob said, did you get it yet?

03-09-2001, 02:26 AM
What size bolt does it come with and also what size blade is on the machine. Does it have a blade saddle or is it friction held. I would assume it comes with a grade 5 at least that is what most manufacturers use as standard hardware.

03-09-2001, 08:54 AM
Go with grade 8. Better to be safe than sorry. Pick'm up at any hardware store or Home Depot.

03-09-2001, 12:47 PM
Grade 8 is a much harder bolt then grade 5 but that is why I asked the question about blade size and whether it is supported in a saddle or friction held. Being a harder steel means a more brittle steel that when shock loaded will crack instead of bend. Basically a harder steel is more brittle and not as flexable. If the bolt is say 1/2" or greater I would be satisfied and feel safer with a grade 5 bolt. I would rather stretch or bend a bolt then break one. Just my thoughts.

Vibe Ray
03-09-2001, 09:09 PM
Yes Mowing machine, that is what I am talking about....Which grade is the best combination of hardness and softness for a mower blade. Grade 8 is not necessarily better than a grade 5 in all applications. That is why I am asking which grade is the best for LAWNMOWERS.
MowingMachine or anyone else who knows, it is for a Lazer HP not quite sure yet what the bolt size is, so just name the best grade for all the possible size bolts for a mower blade bolt
Bob, I should get it in about a week......can't wait!!!

03-09-2001, 09:40 PM
grade 5 for exmark, that is what came from the factory on both of mine. I have bought extra bolts that are 1/2 in. longer for double blades and the regular lenth. IN both cases I purchase grade 5.. I am sure exmark did research so I am sticking with what they use.

Vibe Ray
03-09-2001, 10:00 PM
Thank you Ssouth......anyone else?

Mid Rivers
03-09-2001, 10:32 PM
Use a grade 5. You are not going to bend a bolt when you hit something or if you over tighten it. What is going to happen if you hit something is that you are going to put shear forces on the bolt and a grade 8 bolt is going to shear way before a grade 5. If you over tighten it you are going to put a torsional torque on it and twist a grade 8 off way before you do a grade 5.

I other words there would would be no good reason to put a grade 8 in when you can get a grade 5.;)

Vibe Ray
03-09-2001, 10:36 PM
By the way, How many grades are there? 1-10, right?

03-09-2001, 10:52 PM
I have only heard of 3 grades of Nuts and Bolts, Grades 3,5, and 8. And i bet I can twist a grade 5 1/2" bolt into before I twist a grade 8 1/2" bolt into!!

03-09-2001, 11:00 PM
There is a Grade 9. Sometimes refered to as Lamalloy, a brand name I think. We have used some on my full-time job with the railroad. They are very wear resistant and we use them sometimes on things that are constantly being plunged in to the tie ballast, rock. Those things are ridculously hard, very hard to cut or drill for keeper wires or anything for that matter.

03-09-2001, 11:08 PM
"You could just as easily use grade 8, that's 6 marks on the head" ..... "grade 5 for exmark, that is what came from the factory on both of mine" .......[was just setting here, happened to have one off my 2000 Exmark here setting on the head board of my waterbed (don't ask.....lol) and it has 6 marks on it. Did Exmark decide to go with grade 8 now?]

Vibe Ray
03-09-2001, 11:16 PM
You just had to complicate things didn't you?

[Edited by Vibe Ray on 03-10-2001 at 03:37 AM]

Hardy Enterprises
03-09-2001, 11:52 PM
This is what Marks' Standard Handbook for Mechanical Engineering has listed for bolt grades. There are 10 SAE grades of bolts: 1, 2, 4, 5, 5.1, 5.2, 7, 8, 8.1, 8.2 plus several other ASTM grades that are classified differently. The following are that stats for grade 5 and grade 8 bolts

grade 5 grade 8

Tensile strength (kpsi) 120 150
Yield Strength (kpsi) 92 130

Grade 5 is Medium carbon steel, quenched and tempered
Grade 8 is Medium carbon alloy steel, quenched and tempered

Both grades have been heat treated the primary difference is in the molecular makeup of the steel used there manufacturing.

The short answer is either one should be more than strong enough given the factor safety designed into componets like blade bolts but it is not going to hurt to be on the safe side and use grade 8.


03-10-2001, 02:30 AM
OK didn't want to have to get techincal with mechanical engieering but Hardy Enterprises started it so I will add my 2 cents having a mechanical engineering background. The tensile and yield strengths don't lie a grade 8 bolt clearly can withstand higher stress levels. Here is the catch though, those material properties presented are only good when a static load is applied. In other words a constant none changing load. A lawn mower blade is presented with many cycling loads. Cyclic loading when greater than the endurance limit of the application will lead to fatigue. Once a fatigue crack starts the crack growth rate is significantly higher in the harder material. I would rather have a bolt stretch then break. Stick with the grade 5. Besides that is most likely what was used as standard hardware anyway. Just a thought.

Mid Rivers
03-10-2001, 07:43 PM
MikeGa you are correct, I had my numbers switched around. At least I was consistent about it though. I would still use the stronger (grd 8) bolt.

If the bolt gets a little rust and seizes you are going to twist the weaker (grd 5) one off. I know what you are thinking, you should be taking these off at least every couple days to sharpen the blades but in the winter when they aren't in use who knows what may occur.

If I loose any of mine I will be using a grade 8.

Hardy Enterprises
03-10-2001, 11:36 PM

I too am mechanical engineer, and basically agree with you. In praticality though once you get to the point that you have fatigue cracks it really is not going to mater grade 5 or grade 8 either one is going to break if you hit something solid.


03-11-2001, 02:51 PM
Hardy Enterprises,
I agree with you once there are fatigue cracks it's all over. Failure is going to happen real soon. I guess what I was trying to say is that grade 5 hardware when used in this application may prevent fatigue for a little while longer. This would be due to the slightly larger material grain size allowing more cold working or perment deformation to occur before cracking begins. This would allow the operator to correct for any problems before failure occurs. Then the question is will the grade 5 having less yeild strength be cold worked to the point of failure before the grade 8 with a larger yeild strength has a chance to fatigue. For all practical purposes under normal mowing conditions I would say both would be quite adequate for the job. We are talking about 1/2"-3/4" bolt sizes for most commercial equipment. The normal loads produced by a lawn mower probably wouldn't be great enough to cause failure in the lifetime of the mower. It's those abusive hits every now and then. I have seen 1/2" bolts stretch though, which leads me to believe failure is possible. Just a thought.