Make absolutely sure that the blade nut is really tight not snug.
Check under the cap where the wiper arm bolts to the shaft, as the nut could be loose. It the nut is tight, then remove the cowling. The wiper shaft arms are under the cowling and they are either loose or broken.
if its a rider you remove the deck, turn it over, clamp the old blade in place, & get a large wrench to fit the nut holding on the blade(s). if you have one, use an impact wrench. remove old blade(s) after you remove the nut. take off old blade and put on new one. replace nut and reinstall the deck. if its a pusher just turn mower on its side and use above instructions.hope this helped and good luck
If the blade turns clockwise, then the nut will be left handed thread, meaning to remove it the nut has to be turned to the right .. to tighten then would require turning to the left. To remove the fastener that holds a mower blade on, turn the fastener in the direction the blade rotates to cut the grass. Look at the blade, find the "leading" or sharp edge, and, while holding the blade, turn the fastener in that direction. There is a good reason that this is always true. It has to do with the physics of the machine. Imagine the fastener being "held in place" to stop it from turning as the machine starts. With the machine starting to turn, the fastener will tighten. The last thing you want is to have the fastener become loose when the machine is running. The blade could kill or injure someone if it came off. That's why the fasteners that hold the blades on these machines always come loose when turned in the direction the blade rotates.
It depends on the make of the mower. Some use a central lock nut similar to that used on an F1 wheel. The nut is undone by turn in the opposite direction to the rotation of the blade. To prevent blade turning use a wood chock to stop it as it apply force to the nut. If the nuts jammed use a loosening agent (oil or WD40) first and allow to soak in the other option is 2 to 4 bolts or screws holding the bald to a shaft. these need to be looked for and cleaned as dirty and grass will have blocked then. Remove using a wd40 first to help loosen rusty threads
Remove the mower deck. Block the blade from turning by using a piece of wood wedged between the blade and the mower deck. Loosen the nut on the top of the pulley (May require a cheater bar). Remove the pulley. If it doesn't come off easily, use a 3 armed pully remover. Once the pulley is removed, loosen the four self-tapping bolts that hold the spindle to the bottom of the deck and the spindle will fall loose. Remove the blade. Install the new spindle from below using NEW self-tapping bolts to bolt the new spindle to the deck. Replace the blade, install pulley, block blade from turning then tighten the top nut.
15/16''
Ideally, you want the spindle nut to hold the bearing pack together. Too tight and your bearings wear faster. WAY too tight and the bearings crush. Too loose and the bearings chatter. WAY too loose and the wheel wobbles.
To remove the nut, rotate it counter-clockwise.
Get some leather gloves and a socket wrench set. Lean the mower over by raising one side so you can gain access to the blade. Be careful not to pour out any fluids. Work from the side of the mower that has the grass discharge (the right side). With something holding the mower at a tilt, grasp the blade with a gloved hand and loosen with the correct socket the nut under the blade that is in the center of the blade. As you loosen the nut, the blade will begin to lower. Be careful that you do not lose any of the mechanism, and notice the way it fits together. Just replace in reverse, but a tip is to put a little bit of grease (very little) on the threads of the bolt that holds the blade. Tighten well. I frequently take off my blade and sharpen it. Are you sure that it needs to be replaced? Can it be sharpened? Remember: "Lefty, loosy; righty, tighty."
You have to "safely" prop the front the front on jack stands. The blade is held on with a nut, the easiest way to remove is with a cordless inpact gun, but if unavailable, you can use a socket/ratchet but need to keep the blade from moving.
You probably forgot to put the shear rivets, bolt, key back in depending on the make and model of your lawn mower or you have stripped the bolt.