1340GT: Advice on how to get belt tension correct?

[Bi11Hudson]

Waayyy back, when I went to a military "trade school", USCG EMA, Electrician as a teenager, we were taught a "rule of thumb" system that depended on judgement instead of some gauge that might or might not be available when a repair was made. Assuming a straight edge across the pulleys, could be as simple as a string pulled snug, depress the belt midways to about an inch. The amount of force required is based on thumb pressure. A big, herquy deck ape would tend to set the belt too loose, a "wimpy" navigator type would tend to set the belt too tight. In either case, it works out to "about" 10 pounds on a 3/8 inch belt.

It is a poor (very poor) practice to depend on motor weight to hold belt tension. When the motor bounces from load changes, the tension will vary widely. When I am mounting a motor on a base plate, I release the hold down setscrews and use the above system to judge the belt tension. Essentially the weight of the motor lifting by the belt being pressed "around" 10 pounds. A spring scale can be used but most experienced mechanics judge "that feels about right" to be sufficient. The motor weight screws are then tightened to that position.

Motor drive systems and mounting techniques will vary from machine to machine. And the judgement of a mechanic will vary from person to person. To attempt to quantify an intangable like belt tension depends on many factors. Too loose, it will slip and damage the pulleys and/or belt. Too tight will damage the bearings of the motor and/or the driven load. External factors bear a great deal, is the belt oily or greasy?!? Hot or very cold makes a great deal of difference. Is the belt new or is it a "git by"? To me, "that feels about right" is a judgement call that works.

Your Milage MayWill Vary

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[xr650rRider]

Learning to part is a good time for your belt to be too loose or your going to have one of those awe sh*t moments. You don't have everything pretty much spot on when parting, it can and will go bad quickly. Better the belt to slip than bent twisted parts.

 

[andrewgr]

Waayyy back, when I went to a military "trade school", USCG EMA, Electrician as a teenager, we were taught a "rule of thumb" system that depended on judgement instead of some gauge that might or might not be available when a repair was made. Assuming a straight edge across the pulleys, could be as simple as a string pulled snug, depress the belt midways to about an inch. The amount of force required is based on thumb pressure. A big, herquy deck ape would tend to set the belt too loose, a "wimpy" navigator type would tend to set the belt too tight. In either case, it works out to "about" 10 pounds on a 3/8 inch belt.

It is a poor (very poor) practice to depend on motor weight to hold belt tension. When the motor bounces from load changes, the tension will vary widely. When I am mounting a motor on a base plate, I release the hold down setscrews and use the above system to judge the belt tension. Essentially the weight of the motor lifting by the belt being pressed "around" 10 pounds. A spring scale can be used but most experienced mechanics judge "that feels about right" to be sufficient. The motor weight screws are then tightened to that position.

Motor drive systems and mounting techniques will vary from machine to machine. And the judgement of a mechanic will vary from person to person. To attempt to quantify an intangable like belt tension depends on many factors. Too loose, it will slip and damage the pulleys and/or belt. Too tight will damage the bearings of the motor and/or the driven load. External factors bear a great deal, is the belt oily or greasy?!? Hot or very cold makes a great deal of difference. Is the belt new or is it a "git by"? To me, "that feels about right" is a judgement call that works.

Your Milage MayWill Vary

.

Thanks for the reply!

The 1340 has a grip attached to the motor, that locks the motor in place. If you loosen it, you can use the grip to move the motor up or down. The further down, the more tension on the belt. Once you have the tension you want, you then lock the motor in. So you're using the motor to assist you getting the right tension during setup by acting as a weight, but once you're done with your setup, the tension is no longer related to the motor during actual use. The position is locked in. Sorry if that is obvious, I don't know how common that method is.

 

[pacifica]

Any tips/tricks/advice on how to get the tension on the belt correct?

I measured on my 1340gt and with a 3" sheave on the motor and a 5.2" on gear drive shaft- I have .75 to .80 " movement of the belt. I actually use the free weight of the motor and then lift the motor up about a 1/4". No slippage while working, including parting off . I do use belt dressing which is sticky.

When I made an error and a chuck jaw hit and broke the tip of a parting off blade the belt did slip and saved any further damage. I'm probably safer off not having a big Monarch lathe.

 

[andrewgr]

I measured on my 1340gt and with a 3" sheave on the motor and a 5.2" on gear drive shaft- I have .75 to .80 " movement of the belt. I actually use the free weight of the motor and then lift the motor up about a 1/4". No slippage while working, including parting off . I do use belt dressing which is sticky.

When I made an error and a chuck jaw hit and broke the tip of a parting off blade the belt did slip and saved any further damage. I'm probably safer off not having a big Monarch lathe.

Thanks!

 

[Tipton1965]

By balancing the sheaves, changing to a gates cogged belt direct from gates and reinforcing the stand I was able to almost eliminate vibration that occurred at about 655 to 700 rpm.pms.

What did you do to reinforce the stand?

 

[Jason812]

What did you do to reinforce the stand?

Inquiring minds want to know. I too have a vibration in the same RPM range.

 

[pacifica]

I figured reinforcing the front stand would be best and this is where the vibration is. If you look at the head of 1340 there is quite an overhang where the gearbox is. I believe a weakness in the design. I put 2 machinist jacks under this area and 2 on the motor side to reinforce and support it, but found it made no difference in vibration. I took them out because it took away 2 of the gear positions.

I used 3 pieces of structural steel, all 1/2" thick. A topflat 6" by 24 ", a bottom 6" square and a 6" by 15" wide C channel ( $100). Pieces were welded together($125). I bolted this stand to the side of the cabinet and bottom tray of lathe.
As stated I also balanced all the pulleys, installed a fresh gates belt and centered The transposing gear to the 40t gear(required a .12" spacer).

I also had to align the head but I don"t think that affected vibration.

I stacked solid pieces of steel and brass on this stand. Total added weight is 500 #'s.Also put chucks and anything weighty in the cabinets under the lathe. If you do it before you put the lathe together you can cast concrete or lead into the bottom of the 2 cabinets of the stand.

This reduced vibration .I measured the vibration with an indicator on a stand separate from the lathe and get slightly less than .001" from 655 to 700 and about .0003" at the other rpms. I never did a test of surface finish because there are so many variables involved( tool type, orientation, speeds, feeds, material used, user error and so on), but I have good success parting off, even up to 4+ ".




Front view; top view: and 2 side views with one shot lube for threading gears on upper right.

 

[xr650rRider]

I have the stock cabinets but with wider base added. Vibration was more a function of what chuck was installed than anything else. I can put a ER-40 or 5C collet chuck on and have very little vibration anywhere up to 1800 RPM. Put on the 4 jaw and have pretty good vibration around 700 RPM up to 900 or so. 3 jaw has a little in same range but not as severe as the 4 jaw.

 

[pacifica]

I have the stock cabinets but with wider base added. Vibration was more a function of what chuck was installed than anything else. I can put a ER-40 or 5C collet chuck on and have very little vibration anywhere up to 1800 RPM. Put on the 4 jaw and have pretty good vibration around 700 RPM up to 900 or so. 3 jaw has a little in same range but not as severe as the 4 jaw.

When I did vibration tests I did it without a chuck,nothing on spindle nose.