How do I tighten the tension on the mower deck belt of a JD lt155

Don, there is usually a spring loaded tensioner to do this, the belt may be stretched and needs replaced, or some pulleys actually are now wear items and may need replaced. Check the video links below, some models are adjustable.
"I hope this helped you out, if so let me know by pressing the helpful button. Check out some of my other posts if you need more tips and info."
Mower belt tensioner repair question to the Garage Gang

http://support.mtdproducts.com/app/answers/detail/a_id/563/~/deck-drive-belt-tension-adjustment---riding-mower-(step-thru)

Tightening the belt on John Deere L110 Tractor

Mowing Deck Belt Replacement On Husqvarna LR12 Lawn Tractor

if you dont use kevlar belts,then you are wasting money,they cost more,but,they last ten times longer,use the widest width that it can handle,dont buy from jd,it will cost you 100.00,go to ebay,etc,your tensioner should not be tight enoughto stretch the belt also,here is a jd belt,you can get cheaper just make sure it looks green or red,not rubber black, JOHN DEERE oem Mule Drive Belt TCU13195 737 757 Trak below 030000

OEM Replacement PTO Belt John Deere 737 757 Zero Turn Lawn Mower
JOHN DEERE TRAK 737 757 797 60 DECKS DRIVE BELT TCU13198 12760

try this

How to adjusted Cutting Deck Belt Tension

You do not say which size cutting deck you have. I will be writing about the 48", 3 blade cutting deck

The belt on this deck is self tensioned. The tension is provided by a spring attached to a moveable pulley on the deck. When the belt is in place you can manually force this pulley forward/backward and it will release the tension.

If the belt is off track, place the belt back around all the pulleys in the correct path, lastly force this spring loaded pulley forward/backward (depending on your deck, the direction is not important, the tension is) to get the belt around the pulley. If the belt is placed correctly, when you release the spring loaded pulley it will properly tension the belt.

Typically, these belts should not be jumping off. If yours is coming off frequently the belt may be worn. When they wear they get quite a bit longer, then the tension pulley cannot apply the correct tension. Rarely does the tension spring have to be replaced.

The other problem that causes belt jumping is worn spindles and stationary pulleys. The spindles are the most likely to need replacement. You can buy excellent replacements for JD spindles on Amazon for very reasonable prices. The following spindles are super replacement spindles for the 48" deck. They may very well be the same for smaller and larger decks; I have not researched that. Just search Amazon for Silver Streak # 285851.

Silver Streak # 285851 Spindle Assembly for JOHN DEERE GY21098, JOHN DEERE GY20962,

Product description:
Spindle Assembly / John Deere GY21098 Pack Size:1 JOHN DEERE GY21098, JOHN DEERE GY20962, JOHN DEERE GY20867, JOHN DEERE GY20454 OREGON 82-359, ROTARY 12910, ROTARY 11964 JOHN DEERE D100, D110, D130, D140,D150,D160, LA100, LA105, LA110, LA115, LA120, LA125, LA130,LA135, LA140, LA145, LA155, LA165, X110, X120, X140 Includes grease Zerk, pulley lock nut, blade lock nut, and washer , Height: 7 ", OD: 5 "

You can also find replacements for the flat pulleys on Amazon. If you cannot find what you need there, go to http://www.greenpartstore.com//

Most JD mower decks use a spring loaded tensioner on their belts. Make sure that the belt is threadded thru the pulleys the right way and you may find that considerable effort is required to set the tensioner in position. Be very careful to ensure that if the grip slips the spring flies away from you. I use the hook on a cargo strap to pull mine into position. Normally there is a similar tensioner on the drive belt for the wheels. This is more challenging to get to.

Yes, there is an idler arm with a pulley that presses either inward or outward against the belt to tension it. In some JD mowers, the arm is tensioned by pulling on a long metal rod that latches near the edge of the mower deck, but without the model number I can't be more precise about that. Just look for that extra pulley and arm it's mounted on -- you'll see a tension spring, and you should also see a way to pull the arm away from the spring to place it under tension. Good luck!

Each of the pulleys on top of the deck should have tensioners. These tensioners are usually the type which have a bolt for you to tighten while attached to the pulley. While you tighten the bolt, the pulley puts tension on the belt.

u can change belt yourself need belt size and have to remove cutting deck but check belt tension first if loose with brake off belt stretched out and slipping needs replacement

I am so far from expert...me answering this surprises me. However, I can...JD Small Engines has plenty of diagrams for the 316. Here's how a old single lady changes the belt on her 85 316. There is a bolt running from side to side of a black metal box...on the very front at the bottom of the mower below the grill. If you pull that out, just the front of the mower deck separates from the mower..slip the belt in and around the rear central pulley. Put the pin back in with the belt now around the back pulley and outside the part where the pin goes back in. Get a pneumatic tire jack and jack up the right (your left as you face the front of the machine) front pulley..try to catch the pulley armature or bolt so you don't torque the pulley. This releases the tensioner...thread the belt on the right pulley and reach up under the grill and put the belt over the front engine pulley....now all that's left is to get the belt over the left pulley. At this point, I remember having to either lower the deck to the ground or raise it up all the way...to the ground, I think as it wasn't already in the correct position. Once you start fussing with the left pulley, you will see it is spring loaded and easy to force the belt over if the right pulley is still on the jack. Really important to get the belt inside all the belt guides and lowering the deck before you frustrate yourself with getting the belt over the last pulley. When all is done, release the jack and everything tightens right up.